Question

Click an item in the list or group of pictures at the bottom of the problem and, holding the button down, drag it into the correct position in the answer box. Release your mouse button when the item is place. If you change your mind, drag the item to the trashcan. Click the trashcan to clear all your answers. The sides of an isosceles triangle are 5, 5, and 7. Find the measure of the vertex angle to the nearest degree.

Answer 1

The vertex angle bisector divides the triangle into two congruent right triangles. The sine of half of the vertex angle will be half of the base divided by the side (from the definition of the sine of an angle). Then the total vertex angle is ...
α = 2×arcsin(3.5/5) ≈ 88.9°

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Using the Law of Cosines, the vertex angle can be found as
arccos((7^2 -5^2 -5^2)/(-2*5*5)) = arccos(1/50) ≈ 88.9°