化简求值:已知x、y满足:x 2 +y 2 ﹣4x+6y+13=0,求代数式(3x+y) 2 ﹣3(3x﹣y)(x+y)﹣(x﹣3y)(x+3y)的值.
解:原式=9x 2 +6xy+y 2 ﹣3(3x 2 +3xy﹣xy﹣y 2 )﹣(x2﹣9y 2 ) =9x 2 +6xy+y 2 ﹣9x 2 ﹣6xy+3y 2 ﹣x 2 +9y 2 =﹣x 2 +13y 2 ∵x 2 +y 2 ﹣4x+6y+13=0, ∴(x﹣2) 2 +(y+3) 2 =0, ∴x=2,y=﹣3, 当x=2,y=﹣3时,原式=﹣4+13×9=113. |