Question

cos ( 90o) =

Answer 1

there are a few special angles which you should learn -- they will show up many times
0° sin=0 cos=1 tan=0
30° sin = 1/2 cos = √3/2 tan = 1/√3
45° sin = cos = 1/√2 tan = 1
60° sin = √3/2 cos = 1/2 tan = √3
90° sin = 1 cos = 0 tan = ∞

note that the "co" in cosine means "complementary"
the cosine of an angle is the sine of its complement
same for cosecant and cotangent