Question

已知A=3x的平方-xy+y的平方,B=2x的平方-3xy-2y的平方,求A-[B-(2B+A)]的值,其中x=负2分之一，y=1。

Answer 1

A-[B-(2B+A)]
=A-B+2B+A
=2A+B A=3x的平方-xy+y的平方,B=2x的平方-3xy-2y的平方
=6x²-2xy+2y²+2x²-3xy-2y²
=8x²-5xy x=负2分之一，y=1。
=8*(-1/2)²-5*(-1/2)*1
=2+5/2
=9/2
=4.5

一楼上错的好搞笑。

Answer 2

a-{b-(-2b+a)}=a-{b+2b-a}=a-(3b-a)=a-3b+a=2a-3b
由a=3x^2-xy+y^2
b=2x^2-3xy-2y^
2a=6x^2-2xy+2y^2
3b=6x^2-9xy-6y^2
所以，2a-3b=(6x^2-2xy+2y^2)-(6x^2-9xy-6y^2)=6x^2-2xy+2y^2-6x^2+9xy+6y^2
=7xy+8y^2
又有x=-1/2，y=1，2a-3b =7xy+8y^2=7*1*（-1/2）+8*1=-7/2+8=9/2=4.5

Answer 3

A-[B-(2B+A)]
=A-B+2B+A
=2A+B
=2(3x的平方-xy+y的平方)+(2x的平方-3xy-2y的平方)
=8x²-5xy
=8*(-1/2)²-5*(-1/2)*1
=2+5/2
=9/2

Answer 4

A-[B-(2B+A)]=A-B+(2B+A)]=A-B+2B+A=2A-B

A=3x²-xy+y²
B=2x²-3xy-2y

2A-B=6x²-2xy+2y²-2x²+3xy+2y
=4x²+xy+2y²+2y

x=-1/2，y=1
2A-B=1+1/2+2+2=5+1/2

Answer 5

原式=A-B+2B+A=2A+B
=6x^2-2xy+2y^2+2x^2-3xy-2y^2
=8x^2-5xy
代入数值得:原式=9/2