If a, b, c are vectors such that a+ b+ c= 0 and | a|=7, | b|=5, | c|=3, then the angle bet…

Question

Accepted Answer

$\vec b+\vec c=-\vec a \implies |\vec b+\vec c|^2=|\vec a|^2 \implies 25+9+2\vec b\cdot\vec c=49 \implies 2(5)(3)\cos\theta=15$

$$\cos\theta=\frac12 \implies \theta=60^{\circ}$$

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