. an angle (in this case, angle A). For example, if ∠D measures 10 0°, then m∠D ϭ 10 0°. 1 2 A C D B vertex ray #1 ray #2 –GEOMETRY REVIEW 99 . proportion slope the steepness of a line, as determined by , or ᎏ y x 2 2 Ϫ Ϫ y x 1 1 ᎏ , on a coordinate plane where (x 1 , y 1 ) and (x 2 , y 2 ) are two points on that line solid a three-dimensional. length. major arc an arc greater than or equal to 18 0° midpoint the point at the exact middle of a line segment minor arc an arc less than or equal to 18 0° obtuse angle an angle that measures greater