First slide

Vectors

Question

If vectors P, Q and R have magnitude 5, 12 and 13 units and $\vec{P} + \vec{Q} = \vec{R}$ , the angle between Q and R is

Moderate

A

$\cos^{- 1} \frac{5}{12}$
B

$\cos^{- 1} \frac{5}{13}$
C

$\cos^{- 1} \frac{12}{13}$
D

$\cos^{- 1} \frac{7}{13}$

Solution

$| \vec{P} | = 5, | \vec{Q} | = 12 and | \vec{R} | = 13$
$\cos θ = \frac{Q}{R} = \frac{12}{13}$
$∴ θ = \cos^{- 1} (\frac{12}{13})$

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