True. All isosceles triangles with a 40 degree vertex angle are similar because the corresponding angles are all equal, each triangle having angles of 40-70-70.

An *isosceles triangle* is a triangle with (at least) two equal sides. It has both two equal sides and two equal angles. The name derives from the Greek iso (same) and skelos (leg).

A triangle with all sides equal is called an *equilateral triangle*, and a triangle with no sides equal is called a *scalene triangle*.

An *equilateral triangle* is therefore a special case of an isosceles triangle having not just two, but all three sides and angles equal. Another special case of an isosceles triangle is the *isosceles right triangle*.

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