數學....急呀.....

1. 5a^2/7b 除 20^3/21b
= 5a^2/7b x 21b/20^3
= 5a^2/7b x 21b/8000
= 3a^2/1600
我懷疑你漏了個"a", 是否應是20a^3呢??

2. xy/(x-y) 除 x^2/(y-x)
= xy/(x-y) x (y-x)/x^2
= xy/(x-y) x -(x-y)/x^2
= -y/x

3. 4a+8b/3x-3y 乘 x-y/2b+a
= 4(a+2b)/3(x-y) x (x-y)/(a+2b)
= 4/3

4. 2a-4b/3a+6b 乘 5a+10b/3a-6b
= 2(a-2b)/3(a+2b) x 5(a+2b)/3(a-2b)
= 10/9

5. x(a-b)/3a 除 y(a-b)/12a
= x(a-b)/3a x 12a/y(a-b)
= 4x/y

6. 2x^3/3a 乘 18a^2/x 除 12x/a
= 2x^3/3a x 18a^2/x x a/12x
= xa^2