Find the direction-cosines of a line which makes equal angles wi

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 Multiple Choice QuestionsShort Answer Type

1. If a line makes angles 90°, 135°, 45° with x, y and z-axes respectively, find its direction cosines.
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2.

Can a directed line have direction angles 45°, 60°, 120°?

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3. Can a directed line have direction angles 45°, 45°, 60°?
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4.

A line makes an angle of straight pi over 4 with each of y-axis and z-axis. Find the angle made by it with x-axis. 

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5. Find the direction cosines of x, y and z-axis.
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6. Find the direction-cosines of a line which makes equal angles with the axes. How many such lines are there?


Let α be the angle which the line with all the axes,
∴     its direction-cosines are cos α, cos α, cos α
therefore space space cos squared straight alpha plus cos squared straight alpha plus cos squared straight alpha space equals space 1                                   open square brackets because space space straight l squared plus straight m squared plus straight n squared space equals space 1 close square brackets
therefore space space space 3 space cos space squared space equals space 1 comma space space space space or space space space cos squared straight alpha space equals space 1 third
therefore space space space space space space cos space straight alpha space equals space plus-or-minus space fraction numerator 1 over denominator square root of 3 end fraction
therefore required direction-cosines are plus-or-minus fraction numerator 1 over denominator square root of 3 end fraction comma space plus-or-minus fraction numerator 1 over denominator square root of 3 end fraction comma space plus-or-minus fraction numerator 1 over denominator square root of 3 end fraction
These are four different groups of signs
i.e.,     +, +, +
           +, -, +
           +, +, -
           +, -, -
∴ there are four distinct lines.
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7.

If a line has direction ratios 2, -1, -2, determine its direction cosines. 

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8.

If a line has direction ratios 2, -1, -2, determine its direction cosines. 

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9. If a line has the direction ratios – 18, 12, – 4. then what are its direction cosines?
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10. Find the direction ratios and the direction cosines of the lines joining the pairs of points:
 (–2, 1, 0), (3, 2, 1).
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