x-2y=5 xy=-2.x²+4y²=?(x+2y)²=?x²-3x-6x+6=-2 则2x²-6x+6=?

(x-2y)^2
=x^2+4y^2-4xy
25=x^2+4y^2+8
x^2+4y^2=17
(x+2y)^2
=x^2+4xy+4y^2
=17-8
=9
后面一题请检查下x²-3x-6=-2 则2x²-6x+6=？x^2-3x=42x^2-6x=82x^2-6x+6=14