Question

已知x+y=5,xy=-6,求3x平方+3y平方的值

Answer 1

x+y=5
两边平方
x²+2xy+y²=25
x²+y²=25-2xy=37
所以原式=3(x²+y²)=111

Answer 2

3x(2)+3y(2)
=3[x(2)+y(2)]
=3[x(2)+y(2)+2xy-2xy]
=3[(x+y)(2)-2xy]
=3(x+y)(2)-6xy
把x+y=5,xy=-6代入，得:
=3*5(2)+6*6
=75+36
=111
(
[注:(2)表示平方]