Question : What is the value of $14^{3}+16^{3}+18^{3}+...+30^{3}$?

Option 1: 134576

Option 2: 120212

Option 3: 115624

Option 4: 111672

Correct Answer: 111672

Solution : Given:
$14^{3}+16^{3}+18^{3}+.....+30^{3}$
$= 2^{3}\times7^{3}+2^{3}\times8^{3}+2^{3}\times9^{3}+.....+2^{3}\times15^{3}$
$= 2^{3}(7^{3}+8^{3}+9^{3}+.....+15^{3})$
$= 2^{3}[(1^{3}+2^{3}+3^{3}+4^{3}+....+15^{3})-(1^{3}+2^{3}+3^{3}+4^{3}+5^{3}+6^{3})]$
The sum of cubes of $n$ natural numbers$ =\left[\frac{n(n+1)}{2}\right]^2$
For $n = 6$, we have
$1^{3}+2^{3}+3^{3}+4^{3}+5^{3}+6^{3}=\left[\frac{6(6+1)}{2}\right]^2 = \left[\frac{42}{2}\right]^2 = (21)^{2} =$ 441
For $n = 15$, we have
$1^{3}+2^{3}+3^{3}+4^{3}+....+15^{3}=\left[\frac{15(15+1)}{2}\right]^2 = \left[\frac{240}{2}\right]^2 = (120)^{2} =$ 14400
Now, $2^{3}[(1^{3}+2^{3}+3^{3}+4^{3}+....+15^{3})-(1^{3}+2^{3}+3^{3}+4^{3}+5^{3}+6^{3})]$
$=2^{3}(14400 - 441)$
$= 8 × 13959$
$= 111672$
Hence, the correct answer is 111672.