（3x²-xy）/ （9x²-6xy+y²）
=x(3x-y)/(3x-y)²
=x/(3x-y)
=(1/2)/[3*1/2-(-2/3)]
=(1/2)/[3/2+2/3]
=(1/2)/(13/6)
=1/2*6/13
=3/13