The midpoints of three of the edges of a cube are
labelled A, B, and C as shown in the diagram. What
is the measure of L ABC ?  Select the correct option
A) 90o	B) 105o	C) 120o	D) 135o	E) 150o
Let D be the point on the base of the cube such that CD is perpendicular to   the base. Let s be the length of the sides of the cube. Then AB = BC = AD =   p   2s/2.   Using the Pythagorean Theorem in 4ADC, we get AC =   p   3s/   p2. By the Law of   Cosines AC2 = AB2 +BC2 −2AB · BC · cos(\ABC). We get cos(\ABC) = −1/2, so   L ABC = 120o.   correct answer 120o
<result>C</result>
The midpoints of three of the edges of a cube are
labelled A, B, and C as shown in the diagram. What
is the measure of L ABC ?