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

Let us assume the direction ratios of the line be r1, r2, r3.

Then:

r1 = 2

r2 = –1

r3 = –2

Let us assume the direction cosines for the line be l, m, n

We know that for a line of direction ratios r1, r2, r3 and having direction cosines l, m, n has the following property.   Let us substitute the values of r1, r2, r3 to find the values of .            The direction cosines for the given line is .

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