Bài 1 :
a,
`5/12 + 2/25 - 17/12 + 23/25 + 0,1`
`=` `5/12 - 17/12 + 2/25 + 23/25 + 0,1`
`=` `-1 + 1 + 0,1`
`=` `0,1`
b,
`(1/2 + 3/11 - 10) + (7/11 - 3/2) - (7 - 1/11)`
`=` `1/2 + 3/11 - 10 + 7/11 - 3/2 - 7 + 1/11`
`=` `1/2 - 3/2 - 10 - 7 + 3/11 + 7/11 + 1/11`
`=` `-1 - 17 + 1`
`=` `=-17`
Bài 2 :
a,
`2/5 . x + 3/7 = 3/5`
`2/5 . x = 3/5 - 3/7`
`2/5 . x =6/35`
`->` `x = 3/7`
b,
`2.|2x + 3| + 1 = 5`
`2.|2x + 3| = 5 - 1`
`2. |2x + 3| = 4`
`->` `|2x + 3| =4:2`
`->` `|2x + 3| =2`
`->` `2x + 3 = 2` hoặc `2x + 3 =-2`
`->` `x = -1/2` hoặc `x=-5/2`
c,
`(-3/4)^x = 81/256`
`->` `(-3/4)^x =(±3/4)^4`
`->` `x=4`
Bài 3 :
a,
`x/3 = y/4 => x/3 . 1/5 = y/4 . 1/5 => x/15 = y/20` `(1)`
`y/5 = z/7 => y/5 . 1/4 = z/7 . 1/4 => y/20 = z/28` `(2)`
Từ `(1)` và `(2)` `->` `x/15 = y/20=z/28`
`->` `(2x)/30 = (3y)/60 = z/28`
`text{Theo TCDTSBN ta có: }`
`(2x)/30 = (3y)/60 = z/28 = (2x+ 3y- z)/(30+60-28) = 124/62 = 2`
`x/15 = 2` `->` `x = 30`
`y/20=2` `->` `y=40`
`z/28 = 2` `->` `z=56`
b,
`x/2 = y/3`
Đặt : `x/2 = y/3 = k`
`->` `x=2k ; y = 3k`
`x.y=54`
`->` `2k . 3k = 54`
`->` `6.k^2 =54`
`->` `k^2=9`
`->` `k=±3`
`->` `x=±6` ; `y=±9`
