Cứu hay Hết cứu

`a)` `{x+1}/{x-2}+{2x-3}/{x-2}`

`={x+1+2x-3}/{x-2}`

`={3x-2}/{x-2}`

`b)` `{x+2}/{x^2-9}-1/{x^2+3x}`

`={x+2}/{(x-3)(x+3)}-1/{x(x+3)}`

`={x(x+2)-(x-3)}/{x(x-3)(x+3)}`

`={x^2+2x-x+3}/{x(x-3)(x+3)}`

`={x^2+x+3}/{x(x-3)(x+3)}`

`c)` `{5x}/{42y^2}.{7y}/{x}`

`={5x.7y}/{42y^2.x}`

`=5/{6y}`

`d)` `{1-4x^2}/{x^2+4x}:{2-4x}/{3x}`

`={(1-2x)(1+2x)}/{x(x+4)}.{3x}/{2(1-2x)}`

`={3x(1-2x)(1+2x)}/{2x(x+4)(1-2x)}`

`={3(1+2x)}/{2(x+4)}`

`={3+6x}/{2x+8}`