Tính nhanh

tính nhanh
a) 1/2+ 1/6 + 1/12 + 1/20+ …..+1/9900
b) 1/2+1/4+1/8+1/16+…..+1/1024
c) ( 1+2/3) x ( 1+2/4) x …..x ( 1+2/99)