Exercise 43 Page 381

Looking at the given diagram, we see that LJ≅ LK. We also found that the measure of ∠ JLK=20^(∘) in a previous exercise.

Now, we will remember the classification of triangles.

Classification of Triangles
Scalene Triangle	A scalene triangle is a triangle in which all three sides have different lengths.
Isosceles Triangle	An isosceles triangle is a triangle that has two congruent sides and two base angles with the same measure.
Equilateral Triangle	An equilateral triangle is a triangle in which all the sides are congruent.
Acute Triangle	An acute triangle is a triangle where all angles are less than 90^(∘) or π2.
Obtuse Triangle	An obtuse triangle is a triangle with exactly one an angle whose measure is greater than 90^(∘) or π2.
Right Triangle	A right triangle is a specific type of triangle that contains one angle of 90^(∘).

Since the given triangle has two congruent sides, triangle JKL is an isosceles triangle. We want to find m ∠ JKL. To do so, we will consider the Isosceles Triangle Theorem.