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Explain Solution R.D.Sharma Class 12 Chapter 27 Straight Line in Space Exercise 27.2 Question 22 Maths Textbook Solution.

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Answer  : the angle cannot be made between AB and CD because both are parallel to each other

Given: the coordinates of the points A,B,C,D be (1,2,3);(4,5,7);(-4,3,-3);(2,9,2) respectively. Find angles between the lines AB and CD

Hint: \vec{CD}=2\vec{AB}

Solution:

\begin{aligned} &\vec{A}=(\hat{i}+2 \hat{j}+3 \hat{k}) \\ &\vec{B}=(4 \hat{i}+5 \hat{j}+7 \hat{k}) \\ &\vec{C}=(-4 \hat{i}+3 \hat{j}-6 \hat{k}) \\ \end{aligned}

\begin{aligned} &\vec{D}=(2 \hat{i}+9 \hat{j}+2 \hat{k}) \\ &\overrightarrow{A B}=3 \hat{i}+3 \hat{j}+4 \hat{k} \\ &\overrightarrow{C D}=6 \hat{i}+6 \hat{j}+8 \hat{k} \\ &\therefore C D=2 \overrightarrow{A B} \\ &=2(3 \hat{i}+3 \hat{j}+4 \hat{k}) \\ &=6 \hat{i}+6 \hat{j}+8 \hat{k} \end{aligned}

CD and AB are parallel. So it cannot make any angle

 

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