karush
Gold Member
MHB
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$$a=-4i-2j$$ and $$b=i-7j$$
$$a\cdot b=(-4)(1)+(-2)(-7)=10$$
$$||a||=\sqrt{(-4)^2+(-2)^2}=2\sqrt{5}$$
$$||b||=\sqrt{1^2+(-7)^2}=5\sqrt{2} $$
$$\cos\theta = \frac{10}{10\sqrt{10}}$$ so $$\theta\approx72^o$$
$$a\cdot b=(-4)(1)+(-2)(-7)=10$$
$$||a||=\sqrt{(-4)^2+(-2)^2}=2\sqrt{5}$$
$$||b||=\sqrt{1^2+(-7)^2}=5\sqrt{2} $$
$$\cos\theta = \frac{10}{10\sqrt{10}}$$ so $$\theta\approx72^o$$
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