Question

Identifying the Angles of Rotation
On a coordinate plane, triangle B C D has points (negative 4, 1), (negative 2, 1), (negative 4, 3). Triangle B prime C prime D prime has points (negative 1, negative 4), (negative 1, negative 2), (negative 3, negative 4).

Triangle BCD is rotated counterclockwise to form triangle B’C’D’.

What is the angle of rotation?
45°
90°
180°
360°

Answer 1

90°

Reason: A 90° counterclockwise rotation about the origin sends (x,y) → (−y,x).
Check B(−4,1) → (−1,−4), C(−2,1) → (−1,−2), D(−4,3) → (−3,−4), which match B′, C′, D′.