Exercise 15 Page 498

We want to write $\cos 73^{\circ }$ in terms of sine. To do so, let's recall an important property of the sine and cosine of complementary angles.

The cosine of an acute angle is equal to the sine of its complement.

Keep in mind that complementary angles add up to $90^{\circ }.$ To find the complement of $73,$ we subtract our given angle from $90.$ Now, we can rewrite $\cos 73^{\circ }$ in terms of sine.