Right, there are four different types of triangle. First, here's an isosceles triangle, it has two equal length sides and two equal angles. Oh! Very clever! Now we have an equilateral triangle.

Then angles 𝑎, 𝑏 and 𝑐 must equal 180 as well. This proves that the angles of a triangle must add up to 180 degrees. Add the known angles. Form an equation equal to 360˚. Subtract the ...