Angle between two lines (slopes) (Acute angle θ between two lines with slopes m1,m2 (when product ≠ −1).)

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Acute angle θ between two lines with slopes m1,m2 (when product ≠ −1).

What each symbol means

SymbolWhat it stands for
θAcute angle between them
m1,m2Slopes of the two lines

When to use this

If m1m2=−1, lines perpendicular; formula blows up → θ=90°.