Đáp án:

Bài `17.`

`a,` `15 xx 2121/4343 + 15 xx 222222/434343`

`= 15 xx 21/43 + 15 xx 22/43`

`= 15 xx (21/43 + 22/43)`

`= 15 xx 43/43`

`= 15 xx 1`

`= 15`

`b,` `(16 xx 25 + 44 xx 100)/(29 xx 96 + 142 xx 48)`

`= (4 xx 4 xx 25 + 44 xx 100)/(29 xx 2 xx 48 + 142 xx 48)`

`= (4 xx 100 + 44 xx 100)/(58 xx 48 + 142 xx 48)`

`= (48 xx 100)/(48 xx 200)`

`= 1/2`

`c,` `

`(399xx45+55xx399)/(1995×1996−1991×1995)`

`= (399×(45+55))/(1995×(1996−1991))`

`= (399 xx 100)/(1995 xx 5)`

`= (399 xx 5 xx 20)/(399 xx 5 xx 5)`

`= 20/5`

`= 4`

______________________________

Bài `18.`

`a,` `1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64`

`A = 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64`

`A xx  2 = 2 xx (1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64)`

`A xx 2 = 1 + 1/2 + 1/4 + 1/8 + 1/16 + 1/32`

`A xx 2- A = (1 + 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64) - (1/2 + 1/4 + 1/8 + 1/16 + 1/32 )`

`A = 1 + 1/2 + 1/4 + 1/8 + 1/16 + 1/32- 1/2 - 1/4 - 1/8 - 1/16-1/32`

`A = 1 -1/32`

`A = 31/32`

`b,` `1/(2 xx 3) + 1/(3 xx 4) + 1/(4 xx 5) + 1/(5 xx 6)`

`= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6`

`= 1/2 - 1/6`

`= 3/6 - 1/6`

`= 2/6`

`= 1/3`

$#dairana$