Hello, dapore!

In any triangle with angles and sides

. . .Code:C o *| * * | * * | * b * | * a * | * * | * * | * A o * o * * * o B x D y : - - - - - c - - - - - :

Construct altitude to side

Let

In right triangle .[1]

In right triangle .[2]

Note that: . .[3]

Substitute [1] and [2] into [3]: .