Question

S= 1x20+2x19+3x18+...+20x1 aiuuuda plox

Answer 1

S = 1x20 + 2×19 + 3×18 + ... + 20x1
S = 1×(21-1) + 2×(21-2) + 3×(21-3) + ... + 20×(21-20)
S = (21×1 - 1×1) + (21×2 - 2×2) + (21×3 - 3×3) + ... + (21×20 - 20×20)

Factorizando el 21 te quedaria asi:

S = 21× (1+2+3+...20) - (1x1 + 2×2 + 3×3 + ... + 20×20)
S = 21×(20×21)/2 - (20×21×41)/6
S = 4410 - 2870 = 1540