The y-axis of the graph of the SINE function is a measure of the y-values as you rotate around the unit circle.
angle of rotation.
Ox-values as you rotate around the unit circle.
height of a point above the ground.

1. ∠1 is complementary to ∠2 and ∠3 is complementary to ∠2. subtraction property of equality 2. m∠1 + m∠2 = 90 and m∠3 + m∠2 = 90 substitution 3. m∠1 + m∠2 = m∠3 + m∠2 definition of complementary 4. m∠1 = m∠3 given