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CUET · MATHS · PYQ PAPER 2025

If \(\vec{a}+\vec{b}+\vec{c}=\overrightarrow{0}\) and \(|\vec{a}|=3,|\vec{b}|=5,|\vec{c}|=7\), then the angle between \(\vec{a}\) and \(\vec{b}\) is:

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

Answer & Solution

Correct Answer

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

Step-by-step Solution

Detailed explanation

\(\vec{a}+\vec{b}=-\vec{c}\) \(|\vec{a}+\vec{b}|^2=|-\vec{c}|^2\) \(|\vec{a}|^2+|\vec{b}|^2+2\vec{a}\cdot\vec{b}=|\vec{c}|^2\) \(3^2+5^2+2(3)(5)\cos\theta=7^2\) \(9+25+30\cos\theta=49\) \(34+30\cos\theta=49\) \(30\cos\theta=15\) \(\cos\theta=\frac{1}{2}\) \(\theta=\frac{\pi}{3}\)
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