Supplementary angles: look at the attachment to see the question
1 answer:
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First of all, we can just ignore A, it has no effect but to vertically stretch our cosine.
If it was only , the function would be invertible as long as it's confined between
and
. Now, the argument of our cosine is not
but
. It means that it won't stop at
, but at
.
Another way to think about it, "what should i replace x with so I get inside the cosine?"