`\text{a)}`
Ta có :
`x - y =3`
`=> (x-y)^2 =3^2`
`=> x^2 - 2xy +y^2 = 9`
`=> x^2 - 2 . 10 + y^2 =9`
`=> x^2 -20 +y^2 =9`
`=> x^2 + y^2 = 29`
Vậy `A = 29`
$\\$
`\text{c)}`
`C =x^4+y^4 +2x^2y^2-2x^2y^2`
`=> C = (x^2-y^2)^2 +2x^2y^2`
`=> C = [(x-y)(x+y)]^2 +2x^2y^2`
`=> C = 9(x+y)^2 +2 . 100`
`=> C = 9(x^2 +2xy +y^2) +200`
`=> C = 9(29 + 2 . 10 ) +200`
`=> C = 9 . 49 +200 = 641`
Vậy `C = 641`
