Question

Suppose
cos(u)=3/5
and sin(u) is positive.
sin(u)=
sin(u−π)=
cos(u−π)=
sin(u−π/2)=
cos(u−π/2)=

Answer 1

The co- in cosine means "of the complementary angle." So, cos(x) = sin(π/2-x)

Here,
sin(u) = 4/5

Now just recall the formulas for sum of angles, and you get

sin(u-π) = -sin(u)
cos(u-π) = -cos(u)
sin(u-π/2) = -cos(u)
cos(u-π/2) = sin(u)

Answer 2

ty!