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CUET · MATHS · PYQ PAPER 2025

If \(\vec{a}, \vec{b}\) and \(\vec{c}\) are three vectors such that \(\vec{a}+\vec{b}+\vec{c}=\overrightarrow{0},|\vec{a}|=7,|\vec{b}|=3\) and \(|\vec{c}|=5\), then angle between \(\vec{b}\) and \(\vec{c}\) is

  1. A \(\frac{2 \pi}{3}\)
  2. B \(\frac{\pi}{6}\)
  3. C \(\frac{5 \pi}{6}\)
  4. D \(\frac{\pi}{3}\)
Verified Solution

Answer & Solution

Correct Answer

(D) \(\frac{\pi}{3}\)

Step-by-step Solution

Detailed explanation

\(\vec{a} = -(\vec{b}+\vec{c})\) \(|\vec{a}|^2 = |\vec{b}+\vec{c}|^2\) \(|\vec{a}|^2 = |\vec{b}|^2 + |\vec{c}|^2 + 2|\vec{b}||\vec{c}|\cos\theta\) \(7^2 = 3^2 + 5^2 + 2(3)(5)\cos\theta\) \(49 = 9 + 25 + 30\cos\theta\) \(49 = 34 + 30\cos\theta\) \(15 = 30\cos\theta\)…