## proving identities

1. The problem statement, all variables and given/known data

prove that cos ((pi/2)-x) = sinx

2. Relevant equations

3. The attempt at a solution

i extended it to: (cos pi/2) (cos -x) + (sin pi/2) (sin -x)
=1-sinx
 Recognitions: Homework Help Science Advisor i) cos(a-b)=cos(a)cos(b)+sin(a)sin(b). ii) cos(pi/2)=0. Where did that 1 come from?
 i got the 1 from the sin of pi/2.....isnt that 1?

## proving identities

You cannot expand trig identities like that.

It's not like $$x^2+x=x(x+1)$$

$$\sin{(x+2)}\neq\sin x+\sin2$$

Have you learned the Sum and Differences formula?

You can also prove this through triangles.
 yea we have the sum and difference identities

Recognitions:
Homework Help
 Quote by banfill_89 i got the 1 from the sin of pi/2.....isnt that 1?
Ok, so 1-sinx actually means 1*(-sin(x))?? That isn't the clearest way to write it, wouldn't you agree?? You still have a sign error.
 yea ur right...i forgot the brackets...but it still come sout at -sin(x)......
 oh wait....do i need to include the - on the x?
 cause the subtraction formula is cos ( x - y), and the part of the formula im using is sinxsiny, so do i just need the y number?

Recognitions:
Homework Help
 Quote by banfill_89 yea ur right...i forgot the brackets...but it still come sout at -sin(x)......
Look at the second post. You have a sign error in cosine sum rule.

 Quote by banfill_89 oh wait....do i need to include the - on the x?
Are you familiar with even and odd functions? It's the same with trig functions.

even: f(x)=f(-x)

odd: f(-x)=-f(x)

Recognitions:
Homework Help