# Chapter 10 - Geometry - Chapter Summary, Review, and Test - Review Exercises - Page 680: 15

$m\angle{A} = 51^o$

#### Work Step by Step

The sum of the three interior angles of a triangle is 180 degrees. Angle C is a right angle, so its measure is 90 degrees. Thus, $m\angle{A} + m\angle{B} + m\angle{C} = 180^o \\m\angle{A} + 90^o + 39^o = 180^o \\m\angle{A} + 129^o = 180^o \\m\angle{A} = 180^o - 129^o \\m\angle{A} = 51^o$

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