` 16. ` Gọi số cần tìm là ` a ` 

Theo đề bài ta có: ` [(axx0,5)+0,5]xx0,5-0,5=0,5 `

` => [(axx0,5)+0,5]xx0,5=0,5+0,5=1 `

` => (axx0,5)+0,5=1:0,5=2 `

` => axx0,5=2-0,5=1,5 `

` => a=1,5:0,5=3 `

` => ` Số cần tìm là ` 3 `

` 17. a)(y-1)/5+(y-1)/7+(y-1)/9+...+(y-1)/37+(y-1)/39=0 `

` => (y-1)xx(1/5+1/7+1/9+...+1/37+1/39)=0 `

` 1/5+1/7+1/9+...+1/37+1/39≠0 ` mà ` (y-1)xx(1/5+1/7+1/9+...+1/37+1/39)=0 `

` y-1=0 `

` =>y=1 `

` b)4,34 xx y + 6,76 xx y = 4,995 `

` =>(4,34+6,76)xx y=4,995 `

` => 11,1 xx y=4,995 `

` => y=4,995:11,1=0,45 `

` 18.((3 xx x + 5)-(x+5))/(x-125xx8)=0 `

` => (3 xx x+5)-(x+5)=0 `

` => 3 xx x + 5 - x - 5 = 0 `

` => ( 3 xx x-x)+(5-5)=0 `

` => 2 xx x=0 `

` => x=0 `

` 19. (x-1)/7 + (x-1)/9 + (x-1)/11 + ... + (x-1)/35=0 `

` => (x-1)xx(1/7+1/9+1/11+...+1/35)=0 `

` 1/7+1/9+1/11+...+1/35 ≠0 ` mà ` (x-1)xx(1/7+1/9+1/11+...+1/35)=0 `

` => x-1=0 `

` => x=1 `

` 20. a)2075:5=(x xx 75)/5 +40 `

` => x xx 15 + 40 = 415 `

` => x xx 15 = 415 - 40=375 `

` => x = 375:15=25 `

` b) x + x : 5 xx 7,5 + x : 2 xx 9 = 315 `

` => x + x xx 0,2 xx 7,5 + x xx 0,5 xx 9 =315 `

` => x+x×1,5+x×4,5=315 `

` => x×(1+1,5+4,5)=315 `

` => x×7=315 `

` => x=315:7=45 `

` 21. a) 2,8 × x+1,5=2,7+1,3×x `

` => 2,8 × x - 1,3×x=2,7-1,5 `

` => (2,8-1,3)×x=1,2 `

` => 1,5 × x=1,2 `

` => x=1,2:1,5=0,8 `

 ` b) 1/(2xx3)+1/(3xx4)+1/(4xx5)+...+1/((x-1)xxx)=2/5 `

` => 1/2-1/3+1/3-1/4+1/4-1/5+...+1/(x-1)-1/x=2/5 `

` => 1/2-1/x=2/5 `

` => 1/x=1/2-2/5=5/10-4/10 `

` => 1/x=1/10 `

` => x=10 `