合并同类项:
(1)$\frac {1}{4}ab^{2}-5a^{2}b-\frac {3}{4}a^{2}b+0.75ab^{2}$;
(2)$2x^{2}y-2xy-4xy^{2}+xy+4x^{2}y-3xy^{2}$;
(3)$-\frac {1}{3}xy+(-\frac {2}{5}x^{2})-\frac {1}{2}x^{2}-(-\frac {1}{6}xy)$;
(4)$3(a-b)-4(a-b)-5(a-b)$;
(5)$3(n-m)^{2}-7(n-m)+8(n-m)^{2}+6(n-m)$;
(6)$-3(a-3b)^{3}+2(3b-a)^{2}+4(a-3b)^{2}+2(3b-a)^{3}$.
(1)$\frac {1}{4}ab^{2}-5a^{2}b-\frac {3}{4}a^{2}b+0.75ab^{2}$;
(2)$2x^{2}y-2xy-4xy^{2}+xy+4x^{2}y-3xy^{2}$;
(3)$-\frac {1}{3}xy+(-\frac {2}{5}x^{2})-\frac {1}{2}x^{2}-(-\frac {1}{6}xy)$;
(4)$3(a-b)-4(a-b)-5(a-b)$;
(5)$3(n-m)^{2}-7(n-m)+8(n-m)^{2}+6(n-m)$;
(6)$-3(a-3b)^{3}+2(3b-a)^{2}+4(a-3b)^{2}+2(3b-a)^{3}$.
答案：(1)$ab^{2}-\frac {23}{4}a^{2}b$ (2)$6x^{2}y-xy-7xy^{2}$ (3)$-\frac {1}{6}xy-\frac {9}{10}x^{2}$ (4)$-6a+6b$ (5)$11(n-m)^{2}-n+m$ (6)$-5(a-3b)^{3}+6(a-3b)^{2}$
解析：
(1)解：原式$=(\frac{1}{4}ab^{2}+0.75ab^{2})+(-5a^{2}b-\frac{3}{4}a^{2}b)$
$=ab^{2}-\frac{23}{4}a^{2}b$
(2)解：原式$=(2x^{2}y+4x^{2}y)+(-2xy+xy)+(-4xy^{2}-3xy^{2})$
$=6x^{2}y-xy-7xy^{2}$
(3)解：原式$=-\frac{1}{3}xy-\frac{2}{5}x^{2}-\frac{1}{2}x^{2}+\frac{1}{6}xy$
$=(-\frac{1}{3}xy+\frac{1}{6}xy)+(-\frac{2}{5}x^{2}-\frac{1}{2}x^{2})$
$=-\frac{1}{6}xy-\frac{9}{10}x^{2}$
(4)解：原式$=(3-4-5)(a-b)$
$=-6(a-b)$
$=-6a+6b$
(5)解：原式$=[3(n-m)^{2}+8(n-m)^{2}]+[-7(n-m)+6(n-m)]$
$=11(n-m)^{2}-(n-m)$
$=11(n-m)^{2}-n+m$
(6)解：原式$=-3(a-3b)^{3}+2(a-3b)^{2}+4(a-3b)^{2}-2(a-3b)^{3}$
$=[-3(a-3b)^{3}-2(a-3b)^{3}]+[2(a-3b)^{2}+4(a-3b)^{2}]$
$=-5(a-3b)^{3}+6(a-3b)^{2}$
$=ab^{2}-\frac{23}{4}a^{2}b$
(2)解：原式$=(2x^{2}y+4x^{2}y)+(-2xy+xy)+(-4xy^{2}-3xy^{2})$
$=6x^{2}y-xy-7xy^{2}$
(3)解：原式$=-\frac{1}{3}xy-\frac{2}{5}x^{2}-\frac{1}{2}x^{2}+\frac{1}{6}xy$
$=(-\frac{1}{3}xy+\frac{1}{6}xy)+(-\frac{2}{5}x^{2}-\frac{1}{2}x^{2})$
$=-\frac{1}{6}xy-\frac{9}{10}x^{2}$
(4)解：原式$=(3-4-5)(a-b)$
$=-6(a-b)$
$=-6a+6b$
(5)解：原式$=[3(n-m)^{2}+8(n-m)^{2}]+[-7(n-m)+6(n-m)]$
$=11(n-m)^{2}-(n-m)$
$=11(n-m)^{2}-n+m$
(6)解：原式$=-3(a-3b)^{3}+2(a-3b)^{2}+4(a-3b)^{2}-2(a-3b)^{3}$
$=[-3(a-3b)^{3}-2(a-3b)^{3}]+[2(a-3b)^{2}+4(a-3b)^{2}]$
$=-5(a-3b)^{3}+6(a-3b)^{2}$