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]: .