Ex 27 c, d,e,f va rog urgent dau coroană la primul răspuns complet

Suma Gauss : 1+2+3+...+n = [n(n+1)]/2

c)1+2+3+...+84 = (84×85)/2 = 42×85 = 3570
1+2+3+...+16 = (16×17)/2 = 8×17 = 136
(17+18+19+...+84)/42 = [(84×85)/2 - (16×17)/2] × 1/42 = (42×85 - 8×17)/42 = (3570-136)/42 = 3434/42 = 1717/21

d)676/50 = 338/25

e)(1+2+3+...99) = (99×100)/2 = 99×50 = 4950
(4950-100)/50 = 4850/50 = 97

f)1+2+3+...+449 = (449×450)/2 = 449×225 = 101025
(101025-450)/225 = 447