Question

c =13+14+15+...+42
s 1+3+5+...95+97+99

Answer 1

C = 1+2+..+12+13+...+42-(1+2+..+12)

= 42·43:2 - 12·13:2

= 903 - 78

= 825

S = 1+3+..+99 = 50² = 2500

1+3+..2n-1=n²

2n-1=99=>2n=100=>n=50