Sum of cubes! there is a shortcut

Algebra Level 2

Find the sum of the cubes of the first 1 00 \text100 positive integers.

Bonus : Generalize it for n n


The answer is 25502500.

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5 solutions

Munem Shahriar
Sep 28, 2017

1 3 + 2 3 + 3 3 + 4 3 + 5 3 + 6 3 + + 10 0 3 1^3 +2^3 + 3^3 + 4^3 + 5^3 + 6^3+ \ldots +100^3

n 2 ( n + 1 ) 2 4 \frac{n^2(n+1)^2}{4}

Where n = 100 n = 100

= 10 0 2 ( 100 + 1 ) 2 4 =\frac{100^2(100+1)^2}{4}

= 10 0 2 ( 101 ) 2 4 =\frac{100^2(101)^2}{4}

= 102010000 4 =\frac{102010000}{4}

= 25502500 = \boxed{25502500}

Syed Hamza Khalid
Sep 28, 2017

Raj Bunsha
Dec 9, 2017

Sum of cubes is [n(n+1)/2]^2 Therefore, [100*101/2]^2 =5050^2 =25502500

Sumukh Bansal
Dec 5, 2017

Use 1 3 + 2 3 + 3 3 n 3 = ( 1 + 2 + 3 n ) 2 1^3+2^3+3^3\cdots n^3=(1+2+3\cdots n)^2

1 3 + 2 3 + 3 3 \cdotsn 3 = ( 1 + 2 + 3 \cdotsn ) 2 1^3+2^3+3^3\cdotsn^3=(1+2+3\cdotsn)^2

LaTex error.

Munem Shahriar - 3 years, 6 months ago

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I will correct it.

Sumukh Bansal - 3 years, 6 months ago
Ong Zi Qian
Oct 7, 2017

1 3 + 2 3 + 3 3 + . . . + n 3 = ( 1 + 2 + 3 + 4 + . . . + n ) 2 = n ( n + 1 ) 2 1^3+2^3+3^3+...+n^3 =(1+2+3+4+...+n)^2 =\frac{n(n+1)}{2} ^2 - Since n=100, the answer is 100 ( 101 ) 2 \frac{100(101)}{2} ^2=5050^2=25502500

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