Let’s say that we are talking about the outside angle at the base of the triangle. The inside and outside angles are adjacent angles, their sum is 180 degrees. The inside angle is 138 degrees (180 – 42). We get 2 equal angles of 138 degrees at the base. This is impossible – the sum of the angles in a triangle must be 180 degrees.
Take the angle at a vertex that is not on the bottom. The inner corner is 138.2 remaining inner angles add up to 42 (180 – 138), since each isosceles triangle is 21.
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