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Need solution for RD Sharma Maths Class 12 Chapter 25 Scalar Triple Product Exercise Very Short Answer Question, question 10.

Answers (1)

Answer:

0

Hint:

Use scalar triple product formula. 

Given:

a,b,c are non co planar vectors and\frac{\vec{a} \cdot(\vec{b} \times \vec{c})}{(\vec{c} \times \vec{a}) \cdot \vec{b}}+\frac{\vec{b} \cdot(\vec{a} \times \vec{c})}{\vec{c} \cdot(\vec{a} \times \vec{b})}

Solution:

\frac{\vec{a} \cdot(\vec{b} \times \vec{c})}{(\vec{c} \times \vec{a}) \cdot \vec{b}}+\frac{\vec{b} \cdot(\vec{a} \times \vec{c})}{\vec{c} \cdot(\vec{a} \times \vec{b})}
=\frac{\vec{a} \cdot(\vec{b} \times \vec{c})}{\vec{c} \cdot(\vec{a} \times \vec{b})}+\frac{(\vec{b} \times \vec{a}) \cdot \vec{c}}{\vec{c} \cdot(\vec{a} \times \vec{b})} \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad\left[\begin{array}{l} \because \text { The position of dot and cross can be interchanged } \\ \text { provided that the cyclic order of the vectors remain } \\ \text { same in scalar triple product. } \end{array}\right]

=\frac{\vec{a} \cdot(\vec{b} \times \vec{c})}{(\vec{a} \times \vec{b}) \cdot \vec{c}}+\left\{-\frac{(\vec{a} \times \vec{b}) \cdot \vec{c}}{(\vec{a} \times \vec{b}) \cdot \vec{c}}\right\}\; \; \; \; \; \; \; \; \quad[\because \vec{a} \cdot \vec{b}=\vec{b} \cdot \vec{a} \text { and }(\vec{a} \times \vec{b})=-(\vec{b} \times \vec{a})]

                        \left[\because(\vec{a} \times \vec{b}) \cdot \vec{c}=\left[\begin{array}{lll} \vec{a} & \vec{b} & \vec{c} \end{array}\right]=\vec{a} \cdot(\vec{b} \times \vec{c})\right]

=1-1=0

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