因为 x^ay^2 与(1/3)x^3y^(a + b)是同类项,则相同字母的指数相等,于是有:
a = 3, a + b = 2
解得: a = 3 b = - 1
所以 3 (2a^2 + 5ab - b^2 ) - 2(a^2 + 6ab - b^2)
= 6a^2 + 15ab - 3b^2 - 2a^2 - 12ab + 2b^2
= 4a^2 + 3ab - b^2
把 a = 3, b = - 1代入
原式= 4 × 3^2 + 3×3×(-1) - (-1)^2
= 36 - 27 - 1
= 8