3-11 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} a&=3.90\\ b&=2.70\\ \phi_{ab}&=27.0\degree\\ \end{cases} $$ $$ \begin{aligned} \Ans&=\abs{\vec a\times (\vec b \times \vec a)}\\ &=\abs{\vec a\times \vec c}\\ &=ac ~(\because \vec a \perp \vec c)\\ &=a\cdot\abs{\vec b \times \vec a}\\ &=a\cdot(ba\sin\phi)\\ &=a^2b\sin\phi\\ &=(3.9)^2(2.7)\sin27\degree\\ &\approx 18.64402785280397\\ &\approx 18.6\\ \end{aligned} $$