Question in-con

4.1 Lines, Angles, and Triangles - Angle relationships (parallel lines)
0:00

In convex pentagon A B C D E , segment A B is parallel to segment D E . The measure of angle B is 139 degrees, and the measure of angle D is 174 degrees. What is the measure, in degrees, of angle C ?

Enter your answer:

In convex pentagon A B C D E , segment A B is parallel to segment

Hard-difficulty · SAT Math · Lines, Angles, and Triangles — Angle relationships (parallel lines). Read the question above, select your answer, and check the full explanation below to understand exactly why the correct choice works.

Answer explanation

The correct answer is 47 . It's given that the measure of angle B is 139 degrees. Therefore, the exterior angle formed by extending segment A B at point B has measure 180-139, or 41 , degrees. It's given that segment A B is parallel to segment D E . Extending segment B C at point C and extending segment D E at point D until the two segments intersect results in a transversal that intersects two parallel line segments. One of these intersection points is point B , and let the other intersection point be point X . Since segment A B is parallel to segment D E , alternate interior angles are congruent. Angle CXD and the exterior angle formed by extending segment A B at point B are alternate interior angles. Therefore, the measure of angle CXD is 41 degrees. It's given that the measure of angle D in pentagon A B C D E is 174 degrees. Therefore, angle C D X has measure 180-174, or 6 , degrees. Since angle C in pentagon A B C D E is an exterior angle of triangle C D X , it follows that the measure of angle C is the sum of the measures of angles C D X and CXD. Therefore, the measure, in degrees, of angle C is 6+41, or 47 .

Alternate approach: A line can be created that's perpendicular to segments A B and D E and passes through point C . Extending segments A B and D E at points B and D , respectively, until they intersect this line yields two right triangles. Let these intersection points be point X and point Y , and the two right triangles be triangle BXC and triangle DYC. It's given that the measure of angle B is 139 degrees. Therefore, angle CBX has measure 180-139, or 41 , degrees. Since the measure of angle CBX is 41 degrees and the measure of angle BXC is 90 degrees, it follows that the measure of angle XCB is 180-90-41, or 49 , degrees. It's given that the measure of angle D is 174 degrees. Therefore, angle YDC has measure 180-174, or 6 , degrees. Since the measure of angle YDC is 6 degrees and the measure of angle CYD is 90 degrees, it follows that the measure of angle DCY is 180-90-6, or 84 , degrees. Since angles XCBDCY, and angle C in pentagon A B C D E form segment X Y , it follows that the sum of the measures of those angles is 180 degrees. Therefore, the measure, in degrees, of angle C is 180-49-84, or 47 .