Question

Tính nhanh: 

a, 1/3.4+1/4.5+..........+1/20.21

b, 1/4.6 + 1/6.8 +.............+ 1/30.32

Answer 1

A = 1/3 - 1/4 + 1/4 -1/5 + 1/5 ....-1/20-1/21 

A = 1/3 - 1/21

Answer 2

a,=1/3-1/4+1/4-1/5+.............+1/20-1/21

  =1/3-1/21

=2/7

b,=1/2(1/4.6+1/6.8+............+1/30.32)

    =1/2(1/4-1/6+1/5+1/8+.............+1/30-1/32)

    =1/2(1/4-1/32)

    =1/2.7/32

    =1/64

Answer 3

a) 1/3-1/4+1/4-1/5...........+1/19-1/20+1/20-1/21=1/3-1/21