Q.

The angle between lines 3x+y=1 and x+3y=1 is

1 π6  
2 3π4  
3 5π2  
4 π3  

Ans.

(1)

Given equations of lines are

3x+y=1               ...(i)     and     x+3y=1              ...(ii)

Let m1 and m2 be slopes of (i) and (ii), then

m1=-3,    m2=-13

Let θ be the angle between them, then

tanθ=|m1-m21+m1m2|=|-3+131+3·13|=|-3+123|=13⇒ θ=π6