Tính

A, 31/7 + -5/7 + -8/13 – 1/17

B, -5/7 x 2/11 +-5/7 x 9/11 + 5/2

C, A= 2/11 x 5+2/15x19+2/19x23 +...+ 2/51x55
giúp mk vs ạ