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If \(\vec{a} \,and \,\vec{b}\) are two non-zero vectors then \((\vec{a}-\vec{b})×(\vec{a}+\vec{b})\)=_________(a) \(2(\vec{a}×\vec{b})\)(b) \((\vec{a}×\vec{b})\)(c) –\(4(\vec{a}×\vec{b})\)(d) \(3(\vec{a}×\vec{b})\)This question was posed to me during an online interview.My doubt stems from Product of Two Vectors-2 in portion Vector Algebra of Mathematics – Class 12

Answer»

The correct option is (a) \(2(\vec{a}×\vec{B})\)

To EXPLAIN: Consider \((\vec{a}-\vec{b})×(\vec{a}+\vec{b})\)

=\((\vec{a}-\vec{b})×\vec{a}+(\vec{a}+\vec{b})×\vec{b}\)

=\(\vec{a}×\vec{a}-\vec{b}×\vec{a}+\vec{a}×\vec{b}-\vec{b}×\vec{b}\)

We KNOW that, \(\vec{a}×\vec{a}=0,\vec{b}×\vec{b}=0 \,and \,\vec{a}×\vec{b}=-\vec{b}×\vec{a}\)

∴ \(\vec{a}×\vec{a}-\vec{b}×\vec{a}+\vec{a}×\vec{b}-\vec{b}×\vec{b}=0+2(\vec{a}×\vec{b})+0\)

Hence, \((\vec{a}-\vec{b})×(\vec{a}+\vec{b})=2(\vec{a}×\vec{b})\)



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