Using the formulas: 1 + 2 + ... + n = (n(n + 1)/2 and 12 + 22 + ... + n2 = n(n + 1)(2n + 1)/6
to find the value of: 212 + 222 + ... + 412 + 422:
Note that there are 22 addends.
212 + 222 + ... + 412 + 422 =
(20 + 1)2 + (20 + 2)2 + (20 + 21)2 + (20 + 22)2
Using the binomial expansion:
= ( 202 + 2·20·1 + 12 ) + ( 202 + 2·20·2 + 22 ) + ... + ( 202 + 2·20·21 + 212 ) + ( 202 + 2·20·22 + 222 )
Rearranging:
= [ 202 + 202 + ... + 202 + 202 ] + [ 2·20·1 + 2·20·2 + ... + 2·20·21 + 2·20·22 ] + [ 12 + 22 + ... + 212 + 222 ]
= 22[ 202 ] + [ 2·20·1 + 2·20·2 + ... + 2·20·21 + 2·20·22 ] + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + [ 2·20·1 + 2·20·2 + ... + 2·20·21 + 2·20·22 ] + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + 40 · [ 1 + 2 + ... + 21 + 22 ] + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + 40 · [ 22(1 + 22)/2 ] + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + 40 · [ 253 ] + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + 10120 + [ 12 + 22 + ... + 212 + 222 ]
= 8800 + 10120 + [ 22(22 + 1)(2·22 + 1)/6 ]
= 8800 + 10120 + 3795
= 22715