Question

Номер 8б пожалуйста.

Answer 1

$cos150^{circ }=frac{overline {a}cdot overline {b}}{|overline {a}|cdot |overline {b}|}\\-frac{sqrt3}{2}=frac{-2+2+4z}{sqrt{1+4+16}cdot sqrt{4+1+z^2}}\\frac{4z}{sqrt{21}cdot sqrt{5+z^2}}=-frac{sqrt3}{2}\\-8z=3sqrt7cdot sqrt{5+z^2}\\-8z=3sqrt{35+7z^2}; ; to ; ; -8z>0,; to ; z<0\\64z^2=9(35+7z^2)\\z^2=315\\z=pm sqrt{315}=pm 3sqrt{35}\\Onvet:; z=-3sqrt{35}$