Question

10cos105°/sin15°*cos60°

Answer 1

Условие можно прочитать по-разному

$1)~boldsymbol{dfrac{10cos105textdegree}{sin15textdegree}cdot cos60textdegree=}dfrac{10cos(90textdegree+15textdegree)}{sin15textdegree}cdot dfrac 12=\\=-dfrac{10sin15textdegree}{sin15textdegree}cdot dfrac 12=-10cdot dfrac 12boldsymbol{=-5}$

$2)~boldsymbol{dfrac{10cos105textdegree}{sin15textdegreecdot cos60textdegree}=}dfrac{10cos(90textdegree+15textdegree)}{sin15textdegreecdot frac 12}=\\=-dfrac{10sin15textdegree}{sin15textdegree}cdot dfrac 21=-10cdot 2boldsymbol{=-20}$

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Использована формула приведения

$cos (90textdegree+alpha)=-sin alpha$