合并同类项化简

题目

合并同类项化简
先化简,在求值
2x^2+(-x^2+3xy+2y^2)-(x^2-xy+2y^2).其中x=1/2008,y=2008

答案

2x^2+(-x^2+3xy+2y^2)-(x^2-xy+2y^2)
=(2x^2 - x^2 - x^2) + (3xy - xy) + (2y^2 - 2y^2)
= 2xy
= 2 * (1/2008 * 2008)
= 2*1
= 2