(2^2 + 4^2 + ... + 10^2) - (1^2 + 3^2 + ... + 9^2)
= 2^2 + 4^2 + ... + 10^2 - 1^2 - 3^2 - ... - 9^2
= (2^2 - 1^2) + (4^2 - 3^2) + ... + (10^2 - 9^2)
= (2-1) x (2+1) + (4-3) x (4+3) + ... + (10-9) x (10+9)
= 1 + 2 + 3 + 4 + ... + 9 + 10
= [(1+10) x 10] : 2
= 55