Question

已知,A=4x²+xy+y²,B=x²+xy-5y²,求A-2B的值,其中x=2,y=-2/3。

急！要全面的解答方法。注意格式。要带入代数式。先化简后求值！！好的还有分。

Answer 1

A-2B=(4x²+xy+y²)-2(x²+xy-5y²)
=4x²+xy+y²-2x²-2xy+10y²
=2x²-xy+11y²
x=2，y=-2/3 带入
=8+4/3+11×（-2/3）²
=8+4/3+44/9
=8+56/9
=128/9

Answer 2

A=4x²+xy+y²
B=x²+xy-5y²

A-2B
=4x^2+xy+y^2-2(x^2+xy-5y^2)
=4x^2+xy+y^2-2x^2-2xy+10y^2
=2x^2-xy+11y^2
当x=2,y=-2/3时
原式=2×4+2×2/3+11×4/9
=8+4/3+44/9
=28/3+44/9
=84/9+44/9
=128/9

Answer 3

A-2B
=4x²+xy+y²-2(x²+xy-5y²)
=4x²+xy+y²-2x²-2xy+5y²
=2x²-xy+6y²
=2×4+4/3+6×4/9
=8+4/3+8/3
=8+4
=12