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#### karush

##### Well-known member

- Jan 31, 2012

- 2,771

\(\displaystyle a=-4i-2j\) and \(\displaystyle b=i-7j\)

\(\displaystyle a\cdot b=(-4)(1)+(-2)(-7)=10\)

\(\displaystyle ||a||=\sqrt{(-4)^2+(-2)^2}=2\sqrt{5}\)

\(\displaystyle ||b||=\sqrt{1^2+(-7)^2}=5\sqrt{2} \)

\(\displaystyle \cos\theta = \frac{10}{10\sqrt{10}}\) so \(\displaystyle \theta\approx72^o\)

\(\displaystyle a\cdot b=(-4)(1)+(-2)(-7)=10\)

\(\displaystyle ||a||=\sqrt{(-4)^2+(-2)^2}=2\sqrt{5}\)

\(\displaystyle ||b||=\sqrt{1^2+(-7)^2}=5\sqrt{2} \)

\(\displaystyle \cos\theta = \frac{10}{10\sqrt{10}}\) so \(\displaystyle \theta\approx72^o\)

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