Question

what is the angle between the line x+y=1 and x-y=1?

Answer 1

y=1-x (y=mx+c)  slope is -1  m1=-1
y=x-1   m2=1
angle between lines= tan z= |m1+m2/1-m1m2|
tanz= 1-1/1+1=0/2
 hence z = tan inverse 0=0 degree

Answer 2

we knw that line equ. is y=mx+c, after comparisn those lines, here is m1=-1, m2=1 than angle tanz=m1+m2/1-m1m2 tanz=0, z=0