NDA Math Coordinate System and Straight Line

(A)
Let the slope of the lines be m1 = −1,m2 = −3 and m3=−

1

3

Angle between lines L1 and L2 is tan θ1 = |

−1+3

1+3

|=

1

2

Angle between lines L1 and L3 is tan θ2 = |

−1+ 1 3

1+ 1 3

|=

1

2

Angle between lines L2 and L3 is tan θ2 = |

−3+ 1 3

1+1

|=

4

3

Hence triangle is isosceles