4 + 8 + .. + 2020 =
= 4•(1 + 2 + .. + 505)
= 4•505•506÷2
= 511060
4 + 8 + 12 + ... + 2020 =
4 × ( 505 × 506 : 2 ) =
4 × ( 255530 : 2 ) =
4 × 127765 = 511060
