
now... we have -2/1 but... which one is the negative? well
we know the angle is "<span> π/2 < u < π", that simply means, the 2nd quadrant
well, in the 2nd quadrant, the "x" or adjacent side is negative, so the adjacent side is then -1
now, let us use the pythagorean theorem, to get the opposite side
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now..which one though? well, recall, we're in the 2nd quadrant, in the 2nd quadrant "y" or the opposite side, is positive, so is </span>√(3) then
now... let's plug in those values in the double-angle identities then


simplify away