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