#limits
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$\frac{cos^3(x)-1}{sin^2(2x)}\$
$\= \frac{cos^3(x)-1^3}{(2sin(x)cos(x))^2} \$
$\=\frac{(cos(x)-1) \cdot (cos^2(x) +cos(x)+1)}{4sin^2(x)cos^2(x)}\$
$\= \frac{-(1-cos(x)) \cdot (cos^2(x) +cos(x)+1)}{4(1^2-cos^2(x))cos^2(x)}\$
$\= \frac{-(1-cos(x)) \cdot (cos^2(x) +cos(x)+1)}{4(1-cos(x))(1+cos(x))cos^2(x)}\$
$\= -\frac{cos^2(x) +cos(x)+1}{4cos^2(x)+4cos^3(x)}$
adonhs
$\-\frac{cos^2(0) +cos(0)+1}{4cos^2(0)+4cos^3(0)} = -\frac{3}{8}$
adonhs