a^2+b^2=c^2,a*b=c*ha^2+b^2=(a*b)^2/h^2两边同除以（a*b)^2得到1/(a^2)+1/(b^2)=1/(h^2).
ab=ch=2S三角形ABCa^2*b^2=c^2*h*c^2=a^2+b^2a^2*b^2=(a^2+b^2)h^2两边除以（abh）^2得到1/(a^2)+1/(b^2)=1/(h^2).
