(1）b/a .a/c
=(b×a)/(a×c)
=b/c
（2）x²-4y²/3xy³.xy/x+2y
=(x+2y)(x-2y)(xy)/(3y²)(xy)(x+2y)
=(x-2y)/3y²
(3) 3y/10x .6y²/2x²
=(3y.6y²)/(10x.2x²)
=9y³/10x³
(4)x/x²-1 .x²+x/x²
=x.x(x+1)/(x+1)(x-1)x²
=1/(x-1)