Got this question in textbook assignments:

Prove the cofunction identity using the addition and subtraction formulas.

20. cot (pi/2 - u) = tan u

so to my knowledge, I turn it into:

1 / tan (pi/2 - u) and try to apply subtraction formula but then tan pi/2 isn't defined is it? because 1/0 isn't defined. Is this the case?

[EDIT] I've also run into this now and don't want to spam the forums so I will post here, prove:

sin(pi/2 - x) = sin(pi/2 + x)

I start with RHS = sin(pi/2) cos x + cos (pi/2) sin x

= (1) cos x + (0) sin x

= cos x

Haha this is probly also simple but alas it eludes me...