Step 1:

In right angle triangle , angle , sides .

In right angle triangle, opposite side to the right angle is hypotenuse.

opposite side to the angle is .

From the Pythagorean theorem,

Substitute in above expression.

Side .

Step 2:

From the cosine rule,.

Substitute in above formula.

Step 3:

In a triangle , sum of the three angles is .

Substitute in above formula.