Ta có :
`1^2 + 2^2 + 3^2 + ... + 10^2 = 385`
`S = 2^2 + 4^2 + 6^2 + ... + 20^2`
`= 2^2 + 4^2 + 6^2 + 8^2 + 10^2 + 12^2 + 14^2 + 16^2 + 18^2 + 20^2`
`= ( 1 . 2)^2 + ( 2 . 2)^2 + ( 2 . 3)^2 + ( 2 . 4)^2 + ( 2 . 5)^2 + ( 2 . 6)^2 + ( 2 . 7)^2 + ( 2 .8)^2 + ( 2 . 9)^2 + ( 2 . 10)^2`
`= 1^2 . 2^2 + 2^2 . 2^2 + 2^2 . 3^2 + 2^2 . 4^2 + 2^2 . 5^2 + 2^2 . 6^2 + 2^2 . 7^2 + 2^2 . 8^2 + 2^2 . 9^2 + 2^2 . 10^2`
`= 2^2 . ( 1^2 + 2^2 + 3^2 + 4^2 + 5^2 + 6^2 + 7^2 + 8^2 + 9^2 + 10^2)`
`= 4 . 385`
`= 1540`
Vậy `S = 1540`
`#caty09`
