(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)$m^{2}-3mn^{2}+\frac {1}{2}n^{2}+m^{2}+5mn^{2}-\frac {1}{2}n^{2}$;
(8)$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}$.
(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)$m^{2}-3mn^{2}+\frac {1}{2}n^{2}+m^{2}+5mn^{2}-\frac {1}{2}n^{2}$;
(8)$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}$.
答案：(1)$7a^{2}-9a$ (2)$3xy$ (3)$2a+7b$ (4)$-3x+4y-1$ (5)$-a-2ab$ (6)$2a^{2}+7b^{2}+ab$ (7)$2m^{2}+2mn^{2}$ (8)$3ab^{2}+4$
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：$m^{2}-3mn^{2}+\frac{1}{2}n^{2}+m^{2}+5mn^{2}-\frac{1}{2}n^{2}$
$=(m^{2}+m^{2})+(-3mn^{2}+5mn^{2})+(\frac{1}{2}n^{2}-\frac{1}{2}n^{2})$
$=2m^{2}+2mn^{2}$
(8)解：$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}$
$=(7ab-7ab)+(-3a^{2}b^{2}+3a^{2}b^{2})+(8ab^{2}-5ab^{2})+(7-3)$
$=3ab^{2}+4$
$=(3a^{2}+4a^{2})+(-2a-7a)$
$=7a^{2}-9a$
(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：$m^{2}-3mn^{2}+\frac{1}{2}n^{2}+m^{2}+5mn^{2}-\frac{1}{2}n^{2}$
$=(m^{2}+m^{2})+(-3mn^{2}+5mn^{2})+(\frac{1}{2}n^{2}-\frac{1}{2}n^{2})$
$=2m^{2}+2mn^{2}$
(8)解：$7ab-3a^{2}b^{2}+7+8ab^{2}+3a^{2}b^{2}-3-7ab-5ab^{2}$
$=(7ab-7ab)+(-3a^{2}b^{2}+3a^{2}b^{2})+(8ab^{2}-5ab^{2})+(7-3)$
$=3ab^{2}+4$