Write the direction cosines of a line parallel to z-axis.

Explanation

let`(x-9)/C_1=(y-6)/C_2=(z-6)/C_3=t`

`x=a+c_1t=a`

`y=b+c_2t=b`

`c_1=0,c_2=0,c_3=k`

`sqrt(c_1^2+c_2^2+c_3^2=1`

`sqrt(0+0+k^2)=1`

k=1

the direction cosines of a line parallel to z-axis(0,0,1).