Question Bank - General Aptitude

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Let the sum of the squares of successive integers 0, 1, 2, ..., n - 1, n be denoted by S. Let the sum of the cubes of the same integers be denoted by C. It is desirable that C / S, as n increases in steps of 'unity' from 'zero', is given by the series:\(\frac{0}{1},\frac{3}{3},\frac{9}{5},\frac{{18}}{7},\frac{{30}}{9},\) ...(for n = 0, 1, 2, 3, 4, ...).What will this ratio be for n = 8?

A.
108 / 17
B.
103 / 17
C.
103 / 15
D.
100 / 15

Solution:

Sum of the square of natural number is given as \(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qacaWGtbGaeyypa0ZaaSaaa8aabaWdbiaad6gadaqadaWdaeaapeGa% amOBaiabgUcaRiaaigdaaiaawIcacaGLPaaadaqadaWdaeaapeGaaG% Omaiaad6gacqGHRaWkcaaIXaaacaGLOaGaayzkaaaapaqaa8qacaaI% 2aaaaaaa!4322!S = \frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{6}\)Sum of the cube of natural number is given as,C = \(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qadaqadaWdaeaapeWaaSaaa8aabaWdbiaad6gadaqadaWdaeaapeGa% amOBaiabgUcaRiaaigdaaiaawIcacaGLPaaaa8aabaWdbiaaikdaaa% aacaGLOaGaayzkaaWdamaaCaaaleqabaWdbiaaikdaaaaaaa!3EFC!{\left( {\frac{{n\left( {n + 1} \right)}}{2}} \right)^2}\)Hence,\(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qadaWcaaWdaeaapeGaam4qaaWdaeaapeGaam4uaaaacqGH9aqpdaWc% aaWdaeaapeWaaSaaa8aabaWdbiaad6gadaqadaWdaeaapeGaamOBai% abgUcaRiaaigdaaiaawIcacaGLPaaadaqadaWdaeaapeGaaGOmaiaa% d6gacqGHRaWkcaaIXaaacaGLOaGaayzkaaaapaqaa8qacaaI2aaaaa% WdaeaapeWaaeWaa8aabaWdbmaalaaapaqaa8qacaWGUbWaaeWaa8aa% baWdbiaad6gacqGHRaWkcaaIXaaacaGLOaGaayzkaaaapaqaa8qaca% aIYaaaaaGaayjkaiaawMcaa8aadaahaaWcbeqaa8qacaaIYaaaaaaa% kiabg2da9maalaaapaqaa8qacaaIZaaapaqaa8qacaaIYaaaaiabgE% na0oaalaaapaqaa8qacaWGUbGaaiiOamaabmaapaqaa8qacaWGUbGa% ey4kaSIaaGymaaGaayjkaiaawMcaaaWdaeaapeWaaeWaa8aabaWdbi% aaikdacaWGUbGaey4kaSIaaGymaaGaayjkaiaawMcaaaaaaaa!5DEA!\frac{C}{S} = \frac{{\frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{6}}}{{{{\left( {\frac{{n\left( {n + 1} \right)}}{2}} \right)}^2}}} = \frac{3}{2} \times \frac{{n\;\left( {n + 1} \right)}}{{\left( {2n + 1} \right)}}\)? At n = 1, \(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qadaWcaaWdaeaapeGaam4qaaWdaeaapeGaam4uaaaacqGH9aqpcaaI% WaGaeyypa0ZaaSaaa8aabaWdbiaaicdaa8aabaWdbiaaigdaaaaaaa!3C8E!\frac{C}{S} = 0 = \frac{0}{1}\)? At n = 2, \(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qadaWcaaWdaeaapeGaam4qaaWdaeaapeGaam4uaaaacqGH9aqpcaaI% Xaaaaa!39C6!\frac{C}{S} = 1\)Similarly at n = 8, \(% MathType!MTEF!2!1!+-% feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8% qadaWcaaWdaeaapeGaam4qaaWdaeaapeGaam4uaaaacqGH9aqpcaGG% GcWaaSaaa8aabaWdbiaaiodaa8aabaWdbiaaikdaaaGaey41aq7aaS% aaa8aabaWdbiaaiIdacaGGGcWaaeWaa8aabaWdbiaaiIdacqGHRaWk% caaIXaaacaGLOaGaayzkaaaapaqaa8qadaqadaWdaeaapeGaaGOmai% abgEna0kaaiIdacqGHRaWkcaaIXaaacaGLOaGaayzkaaaaaiabg2da% 9maalaaapaqaa8qacaaIXaGaaGimaiaaiIdaa8aabaWdbiaaigdaca% aI3aaaaaaa!5029!\frac{C}{S} = \;\frac{3}{2} \times \frac{{8\;\left( {8 + 1} \right)}}{{\left( {2 \times 8 + 1} \right)}} = \frac{{108}}{{17}}\)

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