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Approximation of an elliptic integral

I used some nice guesswork to get this formula  
$$\int_0^{\frac{\pi}{2}}\sqrt{a^2\sin^2x+b^2\cos^2x}dx=\frac{ab\pi}{\left(a+b\right)\sin\left(\frac{a\pi}{a+b}\right)}$$  
The comparision for some values of a and b are  
$a=a,b=0, value=a, exact=a$  
$a=1,b=5, value=5.236, exact= 5.2525$  
$a=b, value=\frac{\pi.a}{2}, exact=\frac{\pi.a}{2} $  
You can also check it gives consistent result. Am i right?

https://www.desmos.com/calculator/g0w1nkeotm