Respuestas

2012-10-25T04:38:50+02:00

 

an: a1(N-1)n

a20: 19(20-1)-7

a20: 19(19)-7

a20: 19+-133

a20: -144

 

Suma..

sn(a1+an)/2

S20: 20(19+-144)/2

s20: 20(-125)/2

s20: -2500/2

s20: -1250