Answer:
[tex]\sin{72^\circ}[/tex]
Step-by-step explanation:
In a right triangle, the two acute angles are complementary--they add up to 90 degrees.
Cosine is defined as the adjacent side divided by the hypotenuse. In the attached picture, [tex]\cos{18^\circ}=\frac{a}{h}[/tex].
The other acute angle is 90 - 18 = 72 degrees. Looking at just that angle, side a is the opposite side. So the ratio a/h is opposite over hypotenuse -- the sine ratio.
Therefor, [tex]\cos{18^\circ} = \sin{72^\circ}[/tex]
