1.a:b=4:5 ,a+b=27,求a-b的值.
a=4/5b
a+b=4/5b+b=27
b=15
a-b=4/5b-b=-1/5b=-1/5x15=-3
2.有16开和32开两种课本,他们的长与宽的比相同,其中32开课本的长为184毫米,宽为130毫米,16开课本的宽为184毫米 求16开课本的长.（精确到1毫米）.
184×184÷130≈260毫米
3.已知xy/(x-y)= -1/3,求2x+3xy-2y/x-2xy-y的值~
x-y=-3xy
(2x+3xy-2y)/(x-2xy-y)
=[2(x-y)+3xy]/(x-y-2xy)
=[2*(-3xy)+3xy]/(-3xy-2xy)
=-3xy/(-5xy)
=3/5
4.已知x/2=y/3,你能用两种方法计算x²-y²/x²-2xy-3y²的值吗?
y=3/2x
1) (x²-y²)/(x²-2xy-3y²)
=[x²-(3/2x)²]/[x²-2x*3/2x-3*(3/2x)²]
=-5/4x²/(-35/4x²)
=2/7
2) (x²-y²)/(x²-2xy-3y²)
=(x-y)(x+y)/[(x+y)(x-3y)]
=(x-y)/(x-3y)
=(x-3/2x)/(x-3*3/2x)
=2/7