Question

已知a=3x^2-xy+y^2,b=2x^2-3xy-2y^2,求a-{b-(-2b+a)}的值，其中x=-1/2，y=1

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Answer 1

由题可知：x=-1/2, y=1 ( * 为乘号）
即：a=3*(-1/2)^2-(-1/2*1)+1^2=9/4, b=2(-1/2)^2-3*(-1/2)*1-2*1^2=0
依据数学运算法则中先算小括号，再算中括号，最后算大括号的逻辑
即：a-{b-(-2b+a)} = 9/2=4又1分之2 或 4.5

Answer 2

a-{b-(-2b+a)}=2a+3b=7xy+8y^2=4.5

追问

能写具体点吗？

追答

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