`#Sad`
`a,`
`x^3+y^3`
`= (x+y)(x^2-xy+y^2)`
`\text{→Thay}` `x+y=1` `\text{và}` `xy=-1` `\text{vào ta có:}`
`= 1. (x^2+1+y^2)`
`= x^2+y^2+1`
`= (x^2+2xy+y^2)-2xy+1`
`= (x+y)^2-2xy+1`
`\text{→Thay}` `x+y=1` `\text{và}` `xy=-1` `\text{vào ta có:}`
`= 1^2-2.(-1)+1`
`= 1+2+1`
`= 4`
`\text{Vậy}` `x^3+y^3 = 4`
`b,`
`x^3-y^3`
`= (x-y)(x^2+xy+y^2)`
`\text{Thay}` `x-y=1` `\text{và}` `xy=6` `\text{vào ta có:}`
`= 1. (x^2+6+y^2)`
`= x^2+y^2+6`
`= (x^2-2xy+y^2)+2xy+6`
`= (x-y)^2+2xy+6`
`\text{Thay}` `x-y=1` `\text{và}` `xy=6` `\text{vào ta có:}`
`= 1^2+2.6+6`
`= 1+12+6`
`= 19`
`\text{Vậy}` `x^3-y^3 = 19`
