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If a⃗\vec{a} and b⃗\vec{b} are two vectors such that ∣a⃗∣=10,∣b⃗∣=2|\vec{a}| = 10, |\vec{b}| = 2 and a⃗⋅b⃗=12\vec{a} \cdot \vec{b} = 12, then ∣a⃗×b⃗∣|\vec{a} \times \vec{b}| is equal to

Solution

✅ Correct Option: 1

Given ∣a⃗∣=10|\vec{a}| = 10, ∣b⃗∣=2|\vec{b}| = 2, and a⃗⋅b⃗=12\vec{a} \cdot \vec{b} = 12.

Using the dot product formula a⃗⋅b⃗=∣a⃗∣∣b⃗∣cos⁡θ\vec{a} \cdot \vec{b} = |\vec{a}||\vec{b}|\cos\theta:

12=10×2×cos⁡θ12 = 10 \times 2 \times \cos\theta

12=20cos⁡θ12 = 20\cos\theta

cos⁡θ=1220\cos\theta = \frac{12}{20}

cos⁡θ=35\cos\theta = \frac{3}{5}


Using the Pythagorean identity sin⁡2θ+cos⁡2θ=1\sin^2\theta + \cos^2\theta = 1:

sin⁡2θ=1−cos⁡2θ\sin^2\theta = 1 - \cos^2\theta

sin⁡2θ=1−(35)2\sin^2\theta = 1 - \left(\frac{3}{5}\right)^2

sin⁡2θ=1−925\sin^2\theta = 1 - \frac{9}{25}

sin⁡2θ=1625\sin^2\theta = \frac{16}{25}

sin⁡θ=45\sin\theta = \frac{4}{5}


Using the cross product magnitude formula ∣a⃗×b⃗∣=∣a⃗∣∣b⃗∣sin⁡θ|\vec{a} \times \vec{b}| = |\vec{a}||\vec{b}|\sin\theta:

∣a⃗×b⃗∣=10×2×45|\vec{a} \times \vec{b}| = 10 \times 2 \times \frac{4}{5}

∣a⃗×b⃗∣=20×45|\vec{a} \times \vec{b}| = 20 \times \frac{4}{5}

∣a⃗×b⃗∣=805|\vec{a} \times \vec{b}| = \frac{80}{5}

∣a⃗×b⃗∣=16|\vec{a} \times \vec{b}| = 16

Therefore, ∣a⃗×b⃗∣=16|\vec{a} \times \vec{b}| = 16.

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