Trig Help (Inverse Functions)

Can someone please , please help me with this.

So this involves the use of 2 graphs:

Y = arccos x

y = arctan x

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The question is:

In looking at the sketches you have drawn in question 1, you should see that the inverse relations are not actually functions! Determine through investigation the domain of the trigonometric function, f(x) = sin x, over which the inverse of the trigonometric function, f -1 (x), is also a function. You may use technology if you wish.

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I have no idea how to do it... my teacher didn't explain this to us and he barely teaches us anything in class... I looked through the book but still don't understand it... Can someone please help me, I don't want the answer but how to find the answer, steps and maybe a final answer so I know how to do this from now on.

Thanks :)