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12.合并同类项：
(1) $4x - 9x + 6x - 5x$.
(2) $3a^2b - 2a^2b - \frac{1}{2}a^2b - 5ab^2$.
(3) $a^3b^2 + \frac{1}{2}a^2b - \frac{1}{3}ba^2 + \frac{2}{3}b^2a^3$.
(1) $4x - 9x + 6x - 5x$.
(2) $3a^2b - 2a^2b - \frac{1}{2}a^2b - 5ab^2$.
(3) $a^3b^2 + \frac{1}{2}a^2b - \frac{1}{3}ba^2 + \frac{2}{3}b^2a^3$.
答案
12. (1) $-4x$.
(2) $\frac{1}{2}a^2b - 5ab^2$.
(3) $\frac{5}{3}a^3b^2 + \frac{1}{6}a^2b$.
(2) $\frac{1}{2}a^2b - 5ab^2$.
(3) $\frac{5}{3}a^3b^2 + \frac{1}{6}a^2b$.
13.先合并同类项,再求值:
(1)$2x + 7 + 3x - 2$,其中$x = 2$.
(2)$3x^2 + 4x - 2x^2 + x + x^2 - 3x - 3$,其中$x = 2$.
(3)$2a^2 - 3ab + b^2 - 2ab - 2a^2 + 5ab - 2b + 1$,其中$a = \frac{22}{7},b = -1$.
(4)$\frac{1}{2}(a - b)^2 + \frac{1}{3}(a - b)^2 - 8(a - b)^2 + 7(a - b)^2$,其中$a = -\frac{4}{5},b = -2$.
(1)$2x + 7 + 3x - 2$,其中$x = 2$.
(2)$3x^2 + 4x - 2x^2 + x + x^2 - 3x - 3$,其中$x = 2$.
(3)$2a^2 - 3ab + b^2 - 2ab - 2a^2 + 5ab - 2b + 1$,其中$a = \frac{22}{7},b = -1$.
(4)$\frac{1}{2}(a - b)^2 + \frac{1}{3}(a - b)^2 - 8(a - b)^2 + 7(a - b)^2$,其中$a = -\frac{4}{5},b = -2$.
答案
13. (1) 原式$= 5x + 5$；所求值为 15.
(2) 原式$= 2x^2 + 2x - 3$；所求值为 9.
(3) 原式$= b^2 - 2b + 1$；所求值为 4.
(4) 原式$= -\frac{1}{6}(a - b)$；所求值为 $-\frac{6}{25}$.
(2) 原式$= 2x^2 + 2x - 3$；所求值为 9.
(3) 原式$= b^2 - 2b + 1$；所求值为 4.
(4) 原式$= -\frac{1}{6}(a - b)$；所求值为 $-\frac{6}{25}$.
