Find angles between the lines 
x + y = 1 and x + 
y = 1.
Given lines are 
x + y = 1 and x + 
y = 1.
Slope of line (1) is m1 = -√3, while the slope of line (2) is m2 = -1/√3
Let θ be the angle between two lines
∴ θ = 30°
Thus the angle between the given lines is either 30° or 180° - 30° = 150°
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