Explanation:
if I understand you correctly, then you got
sin(60°) = sqrt(3/2)
and now we have to find cos(60°) based on that.
but : sin(60°) = sqrt(3/4) = sqrt(3)/2
I will continue with that.
one major observation with the trigonometric functions in a norm circle (radius = 1) is that sine and cosine are the legs of a right-angled triangle.
the radius at the angle is the Hypotenuse.
and so, Pythagoras applies
c² = a² + b²
c is the Hypotenuse, a and b are the legs.
so,
radius² = sin²(x) + cos²(x)
and because radius = 1
1² = 1 = sin²(x) + cos²(x)
in our case
1 = sin²(60) + cos²(60)
1 = (sqrt(3)/2)² + cos²(60) = 3/4 + cos²(60)
cos²(60) = 1 - 3/4 = 1/4
cos(60) = sqrt(1/4) = 1/2