It is an application of Pythagorean Theorem
And it is clear that the lower angle is 30 and the apex angle is 60 (this is how always normal TVs look like) ... refer to the image I attached
So, to calculate the width and the length:
[tex] \frac{length}{26} =cos(30)\\~\\length=26*cos(30)=13 \sqrt{3} ~~in[/tex]
[tex] \frac{width}{26} =sin(30)\\~\\width=26*sin(60)=13~~in[/tex]
You can after that round the length to be 22.517 in
Hope you got the idea
