先化简,再求值: ( 1 a - 2 + 1 ) ÷ a 2 - 1 3 a - 6 ,其中 a = tan 60 ° - | - 1 | .
5yx-3x2y-7xy2+6xy-12xy+7xy2+8x2y.
(2a2-1+2a)-3(a-1+a2)