Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 Edited d Authored by Shuli Sog Colordo Sprigs, Colordo shuli_sog@yhoocom Cotets Mth Trick: Metl Clcultio: b cd Mth Competitio Skill: Divisibility by 3 A Problem from Rel Mth Competitio 4 Aswers to All Prctice Problems i Lst Issue 5 Solutios to Cretive Thikig Problems 55 to 57 6 Clues to Cretive Thikig Problems 58 to 6 7 Cretive Thikig Problems 6 to 63 Mth Trick The Trick I the lst issue we preseted how to clculte the followig multiplictios metlly: 6 3 8 4 37 4 47 5 39 45 4 38 These multiplictios re i the geerl form: b cd where, b, c, d d re digits with b close to cd I this short lesso I will preset other short cut through exmples Exmple Metl Clcultio: b cd Clculte 8 3 If we tret 8 s with multiplictio i b Step : Clculte b or b I this exmple, 8 3, the this is or 3 Step : Multiply the result i step by I this exmple, 4 Step 3: Clculte b I this exmple, 3 6 Step 3 Add them this wy: 4 6 4 4 We re doe: 8 3 44 Exmple Clculte 4 3 Tret 4 s 3 with 6 The this is multiplictio i 3 3b Step : Clculte 3 b or 3 b I this exmple, 4 6 or 3 6 6 Step : Multiply the result i step by 3 I this exmple, 6 3 78 Step 3: Clculte b I this exmple, 6 Step 4 Add them: 7 8 7 6 8 We obti 4 3 768 Exmple 3 Clculte 47 5 Tret 47 s 5 with 3 The this is multiplictio i 5 5b Step : Clculte 5 b or 5 b I this exmple, 47 48 or 5 3 48 Step : Multiply the result i step by 5 I this exmple, 48 5 4 Step 3: Clculte b I this exmple, 3 3
Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 Step 4 Add them: 4 3 3 9 7 We obti 47 5 397 Exmple 4 Clculte 68 74 Tret it s multiplictio i 7 7b Step : Clculte 7 b or 7 b I this exmple, 68 4 7 Step : Multiply the result i step by 7 I this exmple, 7 7 54 Step 3: Clculte b I this exmple, 4 8 Step 4 Add them: or 74 7 5 4 8 5 3 We get 68 74 53 Prctice Problems 9 3 7 6 7 8 3 6 4 36 8 37 34 4 45 38 43 3 48 53 67 54 4 56 63 5 68 73 74 64 Mth Competitio Skill Defiitios: Divisibility by Alterte Digit Differece The st, 3 rd, 5 th, digits couted from the right re clled odd plced digits The sum of ll these digits is clled the odd plced digit sum The d, 4 th, 6 th, digits re clled eve plced digits The sum of ll these digits is clled the eve plced digit sum Subtrct the eve plced digit sum from the odd plced digit sum The result is clled the lterte digit differece Exmple Wht is the lterte digit differece of 378? Aswer: The odd plced digit sum is 8 7 5 The eve plced digit sum is 3 5 The the lterte digit differece is 5 5 Exmple Wht is the lterte digit differece of 9876543? Aswer: 5 The odd plced digit sum is 4 6 8 The eve plced digit sum is 3 5 7 9 5 The the lterte digit differece is 5 5 Divisibility by We hve the followig theorem for divisibility by Theorem A umber is divisible by if d oly if the lterte digit differece of the umber is divisible by Note tht is divisible by y turl umber Exmple 3 Is 47,839 divisible? Aswer: Yes The odd plced digit sum is 9 8 4, d the eve plced digit sum is 3 7 The the lterte digit differece is, which is divisible by So 47,839 is divisible by Exmple 4 Is 3456789 divisible? Aswer: No The odd plced digit sum is 9 7 5 3 5, d the eve plced digit sum is 8 6 4 The the lterte digit differece is 5 5, which is ot divisible by So 3456789 is ot divisible by Exmple 5 Is 789 divisible? Aswer: Yes The odd plced digit sum is 4, d the eve plced digit sum is 9 8 7 6 The the lterte digit differece is 4 6 which is divisible by So 789 is divisible by Proof of the Theorem Let N be -digit umber is odd without loss of geerlity Assume Express N i the bse expsio: ' s 999 9 3 9' s 99 Copyright 9 Shuli Sog shuli_sog@yhoocom All Rights Reserved Use with Permissio
Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 m ' s 9' Note tht d 999 m s 9 where m is eve re lwys divisible by Therefore, N is divisible by if d oly if 3, which is the lterte digit differece 3 3, is divisible by Remider upo Divisio by For umber, clculte the lterte digit differece If the result is more th or equl to, clculte the lterte digit differece gi Or subtrct from it If the result is still more th or equl to, subtrct gi util the result is less th If the lterte digit differece is less th, dd to it If the result is still less th, dd gi util the result is lrger th or equl to The fil result is the remider of the umber upo divisio by Exmple 6 Wht is the remider of 9876543 upo divisio by? Aswer: 6 From exmple, the lterte digit differece is 5 5 6 The 6 is the remider Exmple 7 Wht is the remider of 63789 upo divisio by? Aswer: The odd plced digit sum is 9 8 7 6 3, d the eve plced digit sum is 3 6 The lterte digit differece is 3 6 4 4 3 3 The is the swer Exmple 8 Six-digit umber Aswer: 8 Exmples of Problem Solvig 7m 3 is divisible by Fid m The odd plced digit sum is 3 5, d the eve plced digit sum is m 7 8 m The lterte digit differece is 5 8 m 3 m The oly vlue of digit m is 8 such tht 3 m is divisible by Exmple 9 _ Five-digit umber m497 is divisible by where m d re digits with m How my differet pirs of vlues for m d re there? Aswer: 8 The odd plced digit sum is m 6, d the eve plced digit sum is 4 The lterte digit differece is m The m is divisible by So m m could be from to 8, d from to 9 respectively Therefore, there re 8 differet pirs of vlues for m d Exmple! b3987664where d b re digits Fid d b Aswer: d b 4! is divisible by 9 The the digit sum b 48 is divisible by 9 So b 6 or b 5! is divisible by The odd plced digit sum is 5, d the eve plced digit sum is 3 b The lterte digit differece is b The b is divisible by So b or b 9 Solvig for d b we hve d b 4 Exmple Prove tht y six-digit umber bc, bc is divisible by where, b, d c re digits with Proof: The odd plced digit sum is c b, d the eve plced digit sum is b c The lterte digit differece is, which is divisible by Therefore, bc,bc is divisible by Prctice Problems Circle the umbers divisible by : 34 3,53 33 33 777 689 456456 77 75 3876 Fid the remider for ech upo divisio by : 468 3579 3456 9876 9873645 36843 485 6754 444 8743 3 Is 6774999493569 divisible by? 4 Whe 933574654785565 is divided by, wht is the remider? 5 Eight-digit umber Fid m m 37 is divisible by 6 9! 88476993 39b95454366 where d b re digits Fid d b _ 7 Five-digit umber m48 is divisible by where m d re digits How my differet pirs of vlues for m d re there? _ 8 Prove tht y six-digit umber bccb is divisible by where, b, d c re digits with Copyright 9 Shuli Sog shuli_sog@yhoocom All Rights Reserved Use with Permissio 3
Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 A Problem from Rel Mth Competitio Tody s problem comes from MthCouts The problem or similr problem ppered i MthCouts d other mth competitios occsiolly (MthCouts 995 Ntiol Sprit Problem 7) A chord of the lrger of two cocetric circles is tget to the smller circle d mesure 8 iches Fid the umber of squre iches i the re of the shded regio 8 Prctice Problems (MthCout 7 Ntiol Trget Problem 7) Two cocetric circles with rdii of 9 d 9 uits boud shded regio A third circle will be drw with re equl to tht the shded re Wht must the rdius of the third circle be? Express your swer i simplest rdicl form 9 9 Aswer: 8 Theorem I the figure below, C d C re two cocetric circles Chord AB of circle C is tget to circle C Let l be the legth of AB The the re of the rig betwee the two circles re solely determied by l, idepedet of the sizes of the two circles, provided tht l is fixed B l A C (5th AMCB 4 Problem d 55 th AMC B 4 Problem ) A ulus is the regio betwee two cocetric circles The cocetric circles i the figure hve rdii b d c Let OX be rdius of the lrger circle, let XZ be tget to the smller circle t Z, d let OY be the rdius of the lrger circle tht cotis Z Let XZ, y YZ, d e XY Wht is the re of the ulus? Y d Z O c e b X C Proof of the Theorem: Let T be the tget poit, d O be the ceter of the two circles Let R d r be the rdii of circles C d C respectively Drw OA d OT The OT AB d T is the midpoit l of AB Obviously, AT, OT r d OA R I right OTA B l r 4 R So R l l The the re of the rig is R r 4 determied by l solely l C r T O r 4, which is With l 8 the swer to the problem is 8 8 4 R A C A) B) b C) c D) 3 (th AHSME 969 Problem 6) d E) e The re of the rig betwee two cocetric circles is squre iches The legth of chord of the lrger circle tget to the smller circle, i iches, is A) 5 B) 5 C) 5 D) E) 4 (6th AMCA 9 Problem 9) Adre iscribed circle iside regulr petgo, circumscribed circle roud the petgo, d clculted the re of the regio betwee the two circles Bethy did the sme with regulr heptgo (7 sides) The res of the two regios were A d B, respectively Ech polygo hd side legth of Which of the followig is true? 5 5 A) A B B) A B C) A B 49 7 7 49 D) A B E) A B 5 5 Copyright 9 Shuli Sog shuli_sog@yhoocom All Rights Reserved Use with Permissio 4
Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 Aswers to All Prctice Problems i Lst Issue Metl Clcultio 437 43 476 83 864 36 394 7 376 544 368 35 33 4964 4736 Divisibility by 9 The umbers divisible by 9 re: 34 33,3 666 9 5 6 3 7 6 8 3 Yes 4 4 5 9 6 7 7 3 8 7 9 5 A Problem from Rel Mth Competitio 6 divisio by 8 So it is i the leftmost colum Therefore, 3456789 cot be the sum If 3456789 is the sum of the ie umbers i mtrix, 3456789 the cetrl umber must be 3774 9 Sice 3774 3 mod 8, the cetrl umber is t the third colum from the left Therefore, 3456789 c be the sum of the ie umbers i mtrix 56 Coverig with Tetromios You my hve got o s the swer if you hve tried It is correct But re you ble to expli why you cot? I gve the problem to child oce He hd tried little while, d the he sid I kow the swer is o, d I feel tht the T-shped is strge Yes, the T-shped is the odd m Why is it? Color the bord with the stdrd chessbord colorig: Solutios to Cretive Thikig Problems 55 to 57 55 3 x 3 Mtrix Look t the mtrix If we pir the umbers s show, we kow tht the sum is 9 times the cetrl umber The y oe of the followig four tetromioes covers two blck d two white squres: 8 9 3 36 37 38 Therefore, the sum must be multiple of 9 Two umbers i ech pir hs sum of 58 = x 9 95 is ot multiple of 9 So the sum cot be 95 However, if umber is multiple of 9, it is ot ecessrily the sum For exmple, 44 is multiple of 9, but it cot be the sum of the ie umbers i mtrix The reso follows If 44 is the sum of the ie umbers i mtrix, the 44 cetrl umber must be 6 However, 6 is i the 9 rightmost colum If umber is i the leftmost colum, it cot be the cetrl umber of mtrix either Both 3456789 d 9876543 re multiples of 9 If 9876543 is the sum of the ie umbers i mtrix, 9876543 the cetrl umber must be 9739369 9 Recll the shortcut for the divisibility by 8 i Issue 6, Volume 9739369 yields remider of upo There re blck d white squres Wherever you plce these four shpes o the 4 5 rectgle, two ( 4 ) blck d two white squres will be left ucovered These four ucovered squres (two blck d two white) cot be covered by becuse the T-shped tetromio covers three blck d oe white squres, or oe blck d three white squres oly 57 Weighig Met II Agi, let us study from smll umbers We must hve weight of poud for piece of met of poud As we tlked i the Weigh Met I problem (Issue 5, Volume ), we do t wt to mke other weight of poud to weigh piece of met of pouds Isted we would like to mke hevier weight A weight of pouds works Copyright 9 Shuli Sog shuli_sog@yhoocom All Rights Reserved Use with Permissio 5
Shuli s Mth Problem Solvig Colum Volume, Issue Jue, 9 However, we my plce piece of met d weights o oe p If we plce piece of met of pouds with the existig weight of poud together, we would like to mke weight of 3 pouds to blce them With the weight of 3 pouds, we c weigh piece of met of 3 pouds We c weigh piece of met of 4 pouds by combiig the two existig weights To weigh piece of met of 5 pouds, we eed ew weight Sice we my plce the met of 5 pouds with the two existig weights ( poud d 3 pouds) together o oe p, we would like to mke weight of 9 pouds to blce them The we c use the 9-poud weight to blce piece of met of 6 pouds d the 3-poud weight Combie the 9-poud weight d the -poud weight to blce piece of met of 7 pouds d the 3-poud weight Use the 9-poud weight to blce piece of met of 8 pouds d the -poud weight Usig the 9-poud weight we c weigh piece of met of 9 pouds Combiig the 9-poud weight d the -poud weight we c weigh piece of met of pouds Combie the 9-poud weight d the 3-poud weight to blce piece of met of pouds d the -poud weight Combiig the 9-poud weight d the 3-poud weight we c weigh piece of met of pouds Usig ll three existig weights we c weigh piece of met of 3 pouds To weigh piece of met of 4 pouds, we eed ew weight Similrly we would mke weight of 7 pouds to blce the met of 4 pouds d ll three existig weights (-poud, 3-poud, d 9-poud) Now we hve the ptter: the weights re powers of 3 So the fifth weight is 8 pouds Five weights re eough, which re poud, 3 pouds, 9 pouds, 7 pouds, d 8 pouds respectively I fct, with these five weights we c weigh piece of met of up to 3 9 7 8 pouds To weigh piece of met of up to pouds, you my hve differet set of five weights Oe set my be five weights of poud, 3 pouds, 9 pouds, 7 pouds, d 6 pouds respectively Clues to Cretive Thikig Problems 58 to 6 58 A Divisio Fctorize 45 59 Movig Checkers Let me tell you the first move: 6 A Checkerbord Gme For gme strtegy, workig bckwrds very ofte helps Cretive Thikig Problems 6 to 63 6 Four Tromios to Oe Arrge the four L-shped tromios together to mke lrger shpe similr to them 6 Mgic Circles Fill to ito the smll circles such tht the five umbers o ech of the three lrge circles hve sum of 4 Which umber must replce the questio mrk? 63 Two Smrt Studets of Dr Mth Dr Mth hs two smrt studets Al d Bob Dr Mth picks up two itegers m d from to 9 with m Dr Mth clcultes the sum of m d d tells Al the sum, d clcultes the product of m d d tells Bob the product The Al d Bob hve the followig coverstios: Al: I do t kow wht m d re, but I m sure you do t kow either Bob: Now I kow wht m d re Wht does Dr Mth tell Al? First move (Clues d solutios will be give i the ext issues)? Copyright 9 Shuli Sog shuli_sog@yhoocom All Rights Reserved Use with Permissio 6