sat suite question viewer

Geometry and Trigonometry / Lines, angles, and triangles Difficulty: Hard

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 ?

Back question 59 of 79 Next

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 .