Chứng minh rằng: 1/2-1/4+1/8-1/16+1/32-1/64 < 1/3

chứng minh rằng:
a)1/2-1/4+1/8-1/16+1/32-1/64<1/3
b)1/3-2/3^2+3/3^3-4/3^4+...+99/3^99-100/3^100