先化简再求值,3m^2-mn/9m^2-6mn+n^2其中m=3n

问题描述:

先化简再求值,3m^2-mn/9m^2-6mn+n^2其中m=3n

3m^2-mn/9m^2-6mn+n^2
=m(3m-n)/(3m-n)^2
=m/(3m-n)
=3n/(9n-n)
=3/8