Question

 

1.Tính nhanh :7/1x5+7/5x9+7/9x13+7/13x17+7/17x21

Khó quá mình giải mãi không ra kết quả mọi người làm chi tiết giúp mình nhé ... mình cám ơn

Answer 1

gọi tổng này là một số A

ta có  

A=7/4x(1/1x5+1/5x9+1/9x13+1/13x17+1/17x21)

A= 7/4x(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)

A= 7/4x(1-1/21)=7/4x20/21

suy ra A=5/3

Answer 2

7/1×5 + 7/5x9 + 7/9x13 + 7/13x17 + 7/17x21

= 7/4x(4/1x5 + 4/5x9 + 4/9x13 + 4/13x17 + 4/17x21)

= 7/4x(1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + 1/13 - 1/17 + 1/17 - 1/21)

= 7/4x(1-1/20)

= 7/4x19/20

= 133/80

Answer 3

đặt A=7/1X5+7/5X9+7/9X13+7/13X17+7/17X21

Ta có:4A=4x(7/1x5+7/5x9+7/9x13+7/13x17+7/17x21)

4A=7x(4/1x5+4/5x9+4/9x13+4/13x17+4/17x21)

4A=7x(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)

4A=7x(1-1/21)

4A=7x20/21

A=20/3:4

A=20/3 x 1/4

A=5/3

Vậy A=5/3

Answer 4

7/1×5 + 7/5x9 + 7/9x13 + 7/13x17 + 7/17x21

= 7/4x(4/1x5 + 4/5x9 + 4/9x13 + 4/13x17 + 4/17x21)

= 7/4x(1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + 1/13 - 1/17 + 1/17 - 1/21)

= 7/4x(1-1/20)

= 7/4x19/20

= 133/80