Question

sin(x+90) = ?

cos(x+90) = ?

Would the first equal like -sin(x)?
Would the second equal uh -cos(x)?

I'm not quite sure :/

Answer 1

You could actually prove this for yourself by using one of the expansion formulas
recall sin(A+B) = sinAcosB + cosAsinB
so
<b>sin(x+90)</b> = sinx cos90 + cosx sin 90
= sinx (0) + cosx (1)
= <b>cos x</b>
test it with some angles you should know
let x = 30°
sin (30+90)
= sin 120
= √3/2
= cos 30
= √3/2

see if you can find the equivalent of cos(x+90)