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Mathematics · 2026 · 1 mark
CBSE 2026 · Region 3 · Set 1 · Q16
If l_1, m_1, n_1 and l_2, m_2, n_2 are direction cosines of lines L_1 and L_2 respectively and θ is the acute angle between them, then:
If $\displaystyle l_{1}, \mathrm{~m}_{1}, \mathrm{n}_{1}$ and $\displaystyle l_{2}, \mathrm{~m}_{2}, \mathrm{n}_{2}$ are direction cosines of lines $\displaystyle \mathrm{L}_{1}$ and $\displaystyle \mathrm{L}_{2}$ respectively and $\displaystyle \theta$ is the acute angle between them, then :
Official answer
(D)
$\displaystyle \cos \theta=\left|l_{1} l_{2}+\mathrm{m}_{1} \mathrm{~m}_{2}+\mathrm{n}_{1} \mathrm{n}_{2}\right|$
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CBSE Class 12 Mathematics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.