Question

Calcule :

a) 1+3+5+7+9+.............+95+97+99=???????????????????????????

b) 2+4+6+8+10.............+96+98=??????????????????????????????

Answer 1

a)
99=1+(n-1).2
99=1+2n-2
99=2n-1
99+1=2n
2n=100
n=100/2
n=50

sn=(a1+an).n/2
sn=1+99.50/2
sn=1+4950/2
sn=1+2475
sn=2476

b)
98=2+(n-1).2
98=2+2n-2
98=2n
2n=98
n=98/2
n=49

sn=2+98.49/2
sn=2+4802/2
sn=2+2401
sn=2403

Answer 2

a) 2476 ...
b) 2403 ...