Explanation:

If a line has direction ratios of −18,12 and −4, then its direction cosines are

(−18)2+(12)2+(−4)2−18,(−18)2+(12)2+(−4)212,(−4)2+(12)2+(−4)2−4

i.e., 22−18,2212,22−4≡119,116,11−2

Thus, the direction cosines are  −119,116 and 11−2.