BECE Mathematics 2013 Paper

Concept:In an isosceles triangle, the angles opposite the equal sides are equal.Explanation:The given triangle has two sides marked as equal.Hence, it is an isosceles triangle.According to the Isosceles Triangle Theorem, the base angles of an isosceles triangle are equal.In the diagram, one base angle is given as $50^{\circ}$.The angle marked $a$ is the other base angle.Since the base angles are equal, we can write:$$a = 50^{\circ}$$There is no need to use the angle-sum property because the equal sides directly show which angles are equal.Thus, the unknown angle is exactly the same as the given base angle.Answer:The angle marked $a$ is $50^{\circ}$.So, the correct option is B. $50^{\circ}$.