Bài `4:`

Ta có:

`y/(x+y)+(2y^2)/(x^2+y^2)+(4y^4)/(x^4+y^4)+(8y^8)/(x^8-y^8)=4`

`y/(x+y)+(2y^2)/(x^2+y^2)+(4y^4(x^4-y^4)+8y^8)/(x^8-y^8)=4`

`y/(x+y)+(2y^2)/(x^2+y^2)+(4x^4y^4+4y^8)/(x^2-y^8)=4`

`y/(x+y)+(2y^2)/(x^2+y^2)+(4y^4(x^4+y^4))/((x^4-y^4)(x^4+y^4))=4`

`y/(x+y)+(2y^2)/(x^2+y^2)+(4y^4)/(x^2-y^4)=4`

`y/(x+y)+(2y^2(x^2-y^2)+4y^4)/(x^4-y^4)=4`

`y/(x+y)+(2x^2y^2+2y^4)/(x^4-y^4)=4`

`y/(x+y)+(2x^2y^2+2y^4)/(x^4-y^4)=4`

`y/(x+y)+(2y^2(x^2+y^2))/((x^2-y^2)(x^2+y^2))=4`

`y/(x+y)+(2y^2)/(x^2-y^2)=4`

`(y(x-y)+2y^2)/(x^2-y^2)=4`

`(xy+y^2)/(x^2-y^2)=4`

`(y(x+y))/((x-y)(x+y))=4`

`y/(x-y)=4`

`y=4(x-y)`

`y=4x-4y`

`5y=4x` (đpcm)

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