`1`

`a)` `a/b = 3/4 to a/3 = b/4`

Đặt `k=a/3=b/4`

`to a=3k ; b=4k`

Khi đó

`A=(2a-5b)/(a-3b)=(2.3k-4k*5)/(3k-3.4k)=(6k-20k)/(3k-12k)=(-14k)/(-9k)=14/9`

`b)` `a=b-1 to a+1=b`

Khi đó

`B=(3b-a)/(2b+1)=(3(a+1)-a)/(2(a+1)+1)=(3a+3-a)/(2a+2+1)=(2a+3)/(2a+3)=1`

`d)Đặt : k=x/3 = y/5 to x=3k ; y=5k`

`D=(5x^2 + 3y^2)/(10x^2 - 3y^2)`

`D=(5*9k^2 + 3*25k^2)/(10*9k^2-3*25k^2)`

`D=(45k^2 + 75k^2)/(90k^2 - 75k^2)`

`D=(120k^2)/(15k^2)`

`D=8`