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Consider the line x22=2y53,z=1\frac{x-2}{2} = \frac{2y-5}{-3}, z = -1. Then which of the following is/are true?

(A) It has Direction ratios (2, -3, -1)

(B) It has Direction cosines (45,35,15)\left(\frac{4}{5}, \frac{-3}{5}, \frac{-1}{5}\right)

(C) It has Direction ratios (2,32,0)\left(2, \frac{-3}{2}, 0\right)

(D) It has Direction cosines (45,35,0)\left(\frac{4}{5}, \frac{-3}{5}, 0\right)

Choose the correct answer from the options given below:

Solution

Correct Option: 3

The line is given as x22=2y53,z=1\frac{x-2}{2} = \frac{2y-5}{-3}, z = -1

The standard form of a line in 3D is xx0a=yy0b=zz0c\frac{x-x_0}{a} = \frac{y-y_0}{b} = \frac{z-z_0}{c} where (a,b,c)(a, b, c) are the direction ratios.


The y-term can be rewritten:

2y53=2(y52)3\frac{2y-5}{-3} = \frac{2(y-\frac{5}{2})}{-3}

=y5232= \frac{y-\frac{5}{2}}{-\frac{3}{2}}

Since z=1z = -1 is constant, the line doesn't move in the z-direction. This is written as:

z(1)0=z+10\frac{z-(-1)}{0} = \frac{z+1}{0}

When a coordinate is constant, its direction ratio is 0.


The line in standard form:

x22=y5232=z+10\frac{x-2}{2} = \frac{y-\frac{5}{2}}{-\frac{3}{2}} = \frac{z+1}{0}

Direction Ratios =(2,32,0)= \left(2, -\frac{3}{2}, 0\right)


Direction cosines are normalized direction ratios:

Direction cosines=(aa2+b2+c2,ba2+b2+c2,ca2+b2+c2)\text{Direction cosines} = \left(\frac{a}{\sqrt{a^2+b^2+c^2}}, \frac{b}{\sqrt{a^2+b^2+c^2}}, \frac{c}{\sqrt{a^2+b^2+c^2}}\right)

For (a,b,c)=(2,32,0)(a, b, c) = \left(2, -\frac{3}{2}, 0\right):

a2+b2+c2=(2)2+(32)2+(0)2\sqrt{a^2+b^2+c^2} = \sqrt{(2)^2 + \left(-\frac{3}{2}\right)^2 + (0)^2}

=4+94+0= \sqrt{4 + \frac{9}{4} + 0}

=16+94= \sqrt{\frac{16+9}{4}}

=254= \sqrt{\frac{25}{4}}

=52= \frac{5}{2}

Direction cosines:

(252,3252,052)=(45,35,0)\left(\frac{2}{\frac{5}{2}}, \frac{-\frac{3}{2}}{\frac{5}{2}}, \frac{0}{\frac{5}{2}}\right) = \left(\frac{4}{5}, \frac{-3}{5}, 0\right)


(A) Direction ratios (2,3,1)(2, -3, -1) - Incorrect. The z-direction ratio is 0, not -1, and y-direction ratio is 32-\frac{3}{2}, not -3.

(B) Direction cosines (45,35,15)\left(\frac{4}{5}, \frac{-3}{5}, \frac{-1}{5}\right) - Incorrect. The z-direction cosine is 0, not 15-\frac{1}{5}.

(C) Direction ratios (2,32,0)\left(2, \frac{-3}{2}, 0\right) - Correct.

(D) Direction cosines (45,35,0)\left(\frac{4}{5}, \frac{-3}{5}, 0\right) - Correct.


Therefore, options (C) and (D) are true.

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