asin^2θ+bcos^2θ=m,得sin^2θ=（m-b)/(a-b),cos^2θ=(m-a)/(b-a) ( a、b、m、n均不相等);
bsin^2φ+acos^2φ=n,得 sin^2φ=（n-a)/((b-a); cos^2φ=(n-b)/(a-b)
atanθ=btanφ;a^2sin^2θ/cos^2θ=b^2sin^2φ/cos^2φ,
a^2/b^2=(m-a)(n-a)/(n-b)(m-b)=[mn-a(m+n)+a^2]/[mn-b(m+n)+b^2];
[mn-a(m+n)]/a^2=[mn-b(m+n)]/b^2
mn(1/a^2-1/b^2)=(m+n)(1/a-1/b);
1/a+1/b=1/m+1/n