Question

Verify the Identity
cos(x+pi/2)=-sinx

Answer 1

Hey :)
`Cosine Angle Sum Identity` will be useful here!

Answer 2

\[\large\rm \cos\left(\color{royalblue}{A}+\color{orangered}{B}\right)=\cos \color{royalblue}{A} \cos \color{orangered}{B}-\sin \color{royalblue}{A} \sin \color{orangered}{B}\]

Answer 3

\[\large\rm \cos\left(\color{royalblue}{x}+\color{orangered}{\frac{\pi}{2}}\right)=?\]Plug the values into the formula, and simplify! :)

Answer 4

cos(x)cos(pi/2)-sin(x)sin(pi/2)?

Answer 5

Mmm good! :)
Now simplify cos(pi/2) and the sin(pi/2)

Answer 6

-sin(x)

Answer 7

@zepdrix ?

Answer 8

You're trying to get to -sin(x), yes.
But you're missing a step.\[\large\rm \cos\left(x+\frac{\pi}{2}\right)=\cos x \color{orangered}{\cos\frac{\pi}{2}}-\sin x \color{orangered}{\sin\frac{\pi}{2}}\]You have to come up with values for these orange parts.