giúp mk nha các bn

Question

* Tính nhanh:

       

A= 3/3.5+3/5.7+3/7.9+...+3/97.99

B= 21/15.18+21/18.21+21/21.24+...+21/87.90

C= 3/15+3/35+3/63+3/99+3/143

(dấu '' / '' là phân số)

Answer 1

a)A=1-3/5+3/5-3/7+3/7-3/9+...+3/97-3/99

A=1-3/99

A=96/99=32/33

b)bạn làm câu B giống câu A nha,nếu ko biết làm thì bình luận rồi mình giải cho

c)C=3(1/15+1/35+1/63+1/99+1/143)

C=3(1/3.5+1/5.7+1/7.9+1/9.11+1/11.13)

C=1-3/7+3/7-3/9+3/9-3/11+3/11-3/13

C=1-3/13

C=10/13

 

Answer 2

A=3/3*5+3/5*7+3/7*9+...+3/97*99

2/3A=2/3(3/3*5+3/5*7+...+3/97*99)

2/3A=2/3*3/3*5+2/3*3/5*7+...+2/3*3/97*99

2/3A=2/3*5+2/5*7+...+2/97*99

2/3A=1/3-1/5+1/5-1/7+...+1/97-1/99

2/3A=1/3-1/99

2/3A=33/99-1/99

2/3A=32/99

A=32/99÷2/3

A=32/99*3/2

A=32*3/99*2

A=16*1/33*1

A=16/33

B=21/15*18+21/18*21+...+21/87*90

1/7B=1/7(21/15*18+21/18*21+...+21/87*90)

1/7B=1/7*21/15*18+1/7*21/18*21+...+1/7*21/87*90

1/7B=3/15*18+3/18*21+...+3/87*90

1/7B=1/15-1/18+1/18-1/21+…+1/87-1/90

1/7B=1/15-1/90

1/7B=6/90-1/90

1/7B=5/90=1/18

B=1/18÷1/7

B=1/18*7

B=7/18

C=3/15+3/35+3/63+3/99+3/143

C=3/3*5+3/5*7+3/7*9+3/9*11+3/11*13

2/3C=2/3(3/3*5+3/5*7+3/7*9+3/9*11+3/11*13)

2/3C=2/3*3/3*5+2/3*3/5*7+2/3*3/7*9+2/3*3/9*11+2/3*3/11*13

2/3C=2/3*5+2/5*7+2/7*9+2/9*11+2/11*13

2/3C=1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13

2/3C=1/3-1/13

2/3C=13/39-3/39

2/3C=10/39

C=10/39÷2/3

C=10/39*3/2

C=10*3/39*2

C=5*1/13*1

C=5/13