a=1/b,b=1/a,a*b=1/(a*b)(1/a)/(b/3)=(1/a)*(3/b)=3/(a*b)=3*(a*b)(1/b)/(a/5)=(1/b)*(5/a)=5/(a*b)=5*(a*b)(a/4)/(5/b)=(a/4)*(b/5)=(a*b)/20(b/2)/(2/a)=(b/2)*(a/2)=(a*b)/4