合并同类项：
(1) $-3a + 4b + 6a + 5b -12a -9b$；
(2) $-\frac{7}{2}x + 4x^2 +5 -x^2 +0.5x -7$；
(3) $9x^3 -8xy +x^3 -13x^2 -xy$；
(4) $-\frac{1}{3}ab - \frac{1}{4}a^3b + \frac{1}{6}ab + \frac{2}{3}a^3b - \frac{1}{2}ab$；
(5) $-3m^3 -7mn +n^2 -m^3 -mn +4n^2$；
(6) $3x^2y^3z + y^3z -6 + zy^3 +5 -5y^3zx^2$；
(7) $5xy -3x^2y^2 +7 +8xy^2 +5x^2y^2 -7xy$；
(8) $5a^3 -2ab^2 +3a^3 +a^3 +11a^2b -9ab^2$。

(1) $-9a$
(2) $3x^2-3x-2$
(3) $10x^3-13x^2-9xy$
(4) $\frac{5}{12}a^3b-\frac{2}{3}ab$
(5) $-4m^3+5n^2-8mn$
(6) $-2x^2y^3z+2y^3z-1$
(7) $2x^2y^2+8xy^2-2xy+7$
(8) $9a^3-11ab^2+11a^2b$