Are these trig identities correct?

[rocomath]

cos(90+theta) = sin(theta)

sin(90+theta) = -cos(theta)

cos is negative in 2nd quad

sin is positive in 2nd quad

i'm looking at some soln's and i just need clarification, thanks!

 

[symbolipoint]

cos(90+theta) = sin(theta)

sin(90+theta) = -cos(theta)

thanks!

Refer to the unit circle and you can answer this yourself.

 

[cristo]

No, they are not. Do you know an expression for cos(x+90) and sin(x+90), or more generally cos(a+b) and sin(a+b)?

 

[rocomath]

No, they are not. Do you know an expression for cos(x+90) and sin(x+90), or more generally cos(a+b) and sin(a+b)?

lol ah! why didn't it click, thanks again cristo :-]

let me verify these soln's I'm typing up by using the sums/differences or cofunction identities.