No, no, no. Watch your notation! -4/5 and 3/5 are
not angles! They are the ratios.
Look at what Mr. fantastic wrote:
When plugging into the double angle formula
you get this:
This is
not the same as this:
We are saying that
is in Quadrant IV, so
.
You figured out correctly that
.
So we substitute and get
I'll say it again: this is
not the same as this:
I'm sorry if I sound irritated -- I can't tell you how many times I see errors in notation like this. If this was an assignment and I was grading it I would only give partial credit.
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