1.

Evaluate the product \((2\vec{a}+5\vec{b}).(4\vec{a}-5\vec{b})\).(a) \(|\vec{a}|^2+2\vec{a}.\vec{b}-15|\vec{b}|^2\)(b) \(8|\vec{a}|^2+2\vec{a}.\vec{b}-15|\vec{b}|^2\)(c) \(8|\vec{a}|^2-4\vec{a}.\vec{b}-15|\vec{b}|^2\)(d) \(|\vec{a}|^2+\vec{a}.\vec{b}-5|\vec{b}|^2\)The question was asked by my school principal while I was bunking the class.The query is from Product of Two Vectors-2 topic in chapter Vector Algebra of Mathematics – Class 12

Answer» CORRECT answer is (b) \(8|\vec{a}|^2+2\vec{a}.\vec{b}-15|\vec{b}|^2\)

The BEST I can EXPLAIN: To EVALUATE: \((2\vec{a}+5\vec{b}).(4\vec{a}-5\vec{b})\)

=\(2\vec{a}.4\vec{a}-2\vec{a}.5\vec{b}+3\vec{b}.4\vec{a}-3\vec{b}.5\vec{b}\)

=\(8|\vec{a}|^2+2\vec{a}.\vec{b}-15|\vec{b}|^2\)


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