`#tnvt`
`a)x+y=15`
`<=>(x+y)^3=15^3`
`<=>x^3+y^3+3xy(x+y)=3375`
`<=>x^3+y^3+3.56.15=3375`
`<=>x^3+y^3=3375-3.56.15`
`<=>x^3+y^3=855`
Vậy `P=855`
`b)x=8+y`
`<=>x-y=8`
`<=>(x-y)^3=8^3`
`<=>x^3-y^3-3xy(x-y)=512`
`<=>x^3-y^3-3.(-15).8=512`
`<=>x^3-y^3=152`
Vậy `P=152`