Answer:
Option C. cos70°
Explanation:
we know that
If two angles are complementary, then the sine of one angle is equal to the cosine of the other angle and the cosine of one angle is equal to the sine of the other angle
Remember that
Tow angles are complementary , if the sum of their angles is equal to 90 degrees
so
If A+B=90°
then
sin(A)=cos(B)
cos(A)=sin(B)
In this problem we have
A=20°
B=90°-20°=70°
therefore
sin(20°)=cos(70°)
cos(20°)=sin(70°)