Chứng minh: 1/3 + 1/31 + 1/35 + 1/47 + 1/53 + 1/61 < 1/2

Bài 2:CMR,
a.$\frac{1}{3}+\frac{1}{31}+\frac{1}{35}+\frac{1}{47}+\frac{1}{53}+\frac{1}{61}<\frac{1}{2}$
b.$\frac{3}{4}+\frac{8}{9}+\frac{15}{16}+......+\frac{2499}{2500}>48$​