Trigonometric identity needs to be shown.

Show that sin(x+90) = cosx.

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To do this, you'll need to learn a formula:

sin(A+B) = sinAcosB + cosAsinB
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Now to go through with the question:

sin(x+90) = cosx

Now let's take the left hand side:

{sin(A+B) = sinAcosB + cosAsinB}

sin(x + 90) = sinxcos90 + cosxsin90
sin(x + 90) = sinx(0) + cosx(1) [since cos90 is equal to 0 and sin90 is equal to 1]
sin(x + 90) = 0 + cosx
sin(x + 90) = cosx
(Shown)
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I hope this helps!