Let Me Derive The Formula First

Algebra Level 2

X = 101 + 202 + 303 + + 5050 Y = 2 2 + 4 2 + 6 2 + + 10 0 2 \begin{aligned} X &=& 101 + 202 + 303 + \cdots + 5050 \\ Y &=& 2^2+4^2 + 6^2 + \cdots + 100^2 \end{aligned}

Which number is bigger: X X or Y Y ?

X X Y Y They are both equal in value

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2 solutions

Relevant wiki: Sum of n, n², or n³

Consider the ratio below:

X Y = 101 + 202 + 303 + . . . + 5050 2 2 + 4 2 + 6 2 + . . . + 10 0 2 = 101 k = 1 50 k 4 k = 1 50 k 2 = 101 ( 50 ) ( 51 ) 2 × 6 4 ( 50 ) ( 51 ) ( 101 ) = 3 4 < 1 \begin{aligned} \frac{X}{Y} & = \frac{101+202+303+...+5050}{2^2+4^2+6^2+...+100^2} \\ & = \frac{\displaystyle 101 \sum_{k=1}^{50} k}{\displaystyle 4 \sum_{k=1}^{50} k^2} \\ & = \frac{101(50)(51)}{2} \times \frac{6}{4(50)(51)(101)} \\ & = \frac{3}{4} < 1 \end{aligned}

Y > X \implies \boxed{Y > X}


Clarification : From the relevant wiki , we know that 1 + 2 + 3 + + n = 1 2 n ( n + 1 ) 1+2+3+\cdots+n = \dfrac12 n(n+1) and 1 2 + 2 2 + 3 2 + + n 2 = 1 6 n ( n + 1 ) ( 2 n + 1 ) 1^2+2^2+3^2+\cdots+n^2=\dfrac16 n(n+1)(2n+1) .

Paola Ramírez
May 2, 2016

X = 101 ( 1 + 2 + 3 + 4 + 5... + 50 X=101(1+2+3+4+5...+50

X = 101 ( 50 ) ( 51 ) 2 = 128775 X=101\frac{(50)(51)}{2}=\boxed{128775}

Y = 4 ( 1 2 + 2 2 + 3 2 + . . . + 5 0 2 ) Y=4(1^2+2^2+3^2+...+50^2)

Y = 4 ( 50 ( 51 ) ( 101 ) 6 ) = 171700 Y=4(\frac{50(51)(101)}{6})=\boxed{171700}

Y > X \therefore Y>X

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