7y/2x*根号下(1/y^2)-(x/y^3) 且(y>x>0)
=7y/2x*1/y√(1-x/y)
=7/2x√(y-x)/y
=7/2xy√(y²-xy)
y/(3x-y)*根号下（9x^3y-6x^2y^2+xy^3)/y^2 且（3x>y>0）
=y/(3x-y)*1/y√(9x³y-6x²y²+xy³)
=1/(3x-y)√xy(9x²-6xy+y²)
=1/(3x-y)√xy(3x-y)²
=1/(3x-y)*(3x-y)√xy
=√xy