Isosceles triangle, through side and angle

Knowing the lateral side of the isosceles triangle and the angle at the base, you can find the third side. To do this, draw a height falling on the base, which will divide the isosceles triangle into two identical right-angled triangles, and the base also into two equal parts. Half of the base from trigonometric relations in the new triangle will be equal to the product of the side (hypotenuse) and the cosine of the angle at the base.