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Lines, angles, and triangles SAT question

Math · Geometry and Trigonometry · Easy · Answer it here, explanation included.

The figure presents triangle A B C, where side A C is horizontal, vertex A is to the left of vertex C, vertex B is above side A C, and the measure of angle A B C is labeled x degrees. Side A C extends horizontally to the right to point D, and the angle, B C D, that is above line segment C D and to the right of side B C has a measure of one hundred ten degrees.

In the given figure, side A C extends to point D. If the measure of angle B A C is equal to the measure of angle B C A, what is the value of x ?

ExplanationShow

Choice D is correct. Since angle B C D and angle B C A form a linear pair of angles, their measures sum to 180°. It’s given that the measure of angle B C D is 110°. Therefore, 110 degrees plus angle B C A, equals 180 degrees. Subtracting 110° from both sides of this equation gives the measure of angle B C A as 70°. It’s also given that the measure of angle B A C is equal to the measure of angle B C A. Thus, the measure of angle B A C is also 70°. The measures of the interior angles of a triangle sum to 180°. Thus, 70 degrees, plus 70 degrees, plus x degrees, equals 180 degrees. Combining like terms on the left-hand side of this equation yields 140 degrees plus x degrees, equals 180 degrees. Subtracting 140° from both sides of this equation yields x degrees equals 40 degrees, or x equals 40.

Choice A is incorrect. This is the value of the measure of angle B C D. Choice B is incorrect. This is the value of the measure of each of the other two interior angles, angle B C A and angle B A C. Choice C is incorrect and may result from an error made when identifying the relationship between the exterior angle of a triangle and the interior angles of the triangle.

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