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If ∣a⃗∣=8|\vec{a}| = 8, ∣b⃗∣=3|\vec{b}| = 3 and ∣a⃗×b⃗∣=12|\vec{a} \times \vec{b}| = 12, then the value of a⃗⋅b⃗\vec{a} \cdot \vec{b} is

Solution

✅ Correct Option: 2

Use the identity ∣a⃗∣2∣b⃗∣2=∣a⃗×b⃗∣2+(a⃗⋅b⃗)2|\vec{a}|^2 |\vec{b}|^2 = |\vec{a} \times \vec{b}|^2 + (\vec{a} \cdot \vec{b})^2. Substituting: 64⋅9=144+(a⃗⋅b⃗)264 \cdot 9 = 144 + (\vec{a} \cdot \vec{b})^2, so (a⃗⋅b⃗)2=576−144=432(\vec{a} \cdot \vec{b})^2 = 576 - 144 = 432. Therefore a⃗⋅b⃗=±432=±123\vec{a} \cdot \vec{b} = \pm \sqrt{432} = \pm 12\sqrt{3}.

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