合并同类项：
(1)$3a^{2}-2a+4a^{2}-7a$； (2)$xy+7xy-5xy$；
(3)$-5a-2b+7a+9b$； (4)$2x-7y-5x+11y-1$；
(5)$-3a+2ab-4ab+2a$； (6)$4a^{2}+3b^{2}+2ab-2a^{2}+4b^{2}-ab$；
(7)$\frac {1}{2}m^{2}-3mn^{2}+4n^{2}+\frac {1}{2}m^{2}+5mn^{2}-4n^{2}$； (8)$-5m^{2}n+4mn^{2}-2mn+6m^{2}n+3mn$；
(9)$7a^{2}-2ab+b^{2}-5a^{2}-b^{2}-2a^{2}-ab$；
(10)$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}$.
答案：(1)7a² - 9a (2)3xy (3)2a+7b (4)-3x+4y-1 (5)-a-2ab (6)2a²+7b²+ab (7)m²+2mn² (8)m²n+4mn²+mn (9)-3ab (10)3ab²+4
解析：
(1)解：$3a^{2}-2a+4a^{2}-7a=(3a^{2}+4a^{2})+(-2a-7a)=7a^{2}-9a$
(2)解：$xy+7xy-5xy=(1+7-5)xy=3xy$
(3)解：$-5a-2b+7a+9b=(-5a+7a)+(-2b+9b)=2a+7b$
(4)解：$2x-7y-5x+11y-1=(2x-5x)+(-7y+11y)-1=-3x+4y-1$
(5)解：$-3a+2ab-4ab+2a=(-3a+2a)+(2ab-4ab)=-a-2ab$
(6)解：$4a^{2}+3b^{2}+2ab-2a^{2}+4b^{2}-ab=(4a^{2}-2a^{2})+(3b^{2}+4b^{2})+(2ab-ab)=2a^{2}+7b^{2}+ab$
(7)解：$\frac{1}{2}m^{2}-3mn^{2}+4n^{2}+\frac{1}{2}m^{2}+5mn^{2}-4n^{2}=(\frac{1}{2}m^{2}+\frac{1}{2}m^{2})+(-3mn^{2}+5mn^{2})+(4n^{2}-4n^{2})=m^{2}+2mn^{2}$
(8)解：$-5m^{2}n+4mn^{2}-2mn+6m^{2}n+3mn=(-5m^{2}n+6m^{2}n)+4mn^{2}+(-2mn+3mn)=m^{2}n+4mn^{2}+mn$
(9)解：$7a^{2}-2ab+b^{2}-5a^{2}-b^{2}-2a^{2}-ab=(7a^{2}-5a^{2}-2a^{2})+(b^{2}-b^{2})+(-2ab-ab)=-3ab$
(10)解：$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}=(7ab-7ab)+(-3a^{2}b^{2}+3a^{2}b^{2})+(8ab^{2}-5ab^{2})+(7-3)=3ab^{2}+4$
