Hello,

For that, I will use a rough method, using more algebra than geometry.

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Because I don't remember the formula for , I'll show it.

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For that I will need to find the formula (which I don't remember either) of tan(c+d) :

Let's multiply by

This gives :

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Therefore, we have :

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Now let's go back to the problem.

So we have

(formula above).

Therefore,