The angle θ between two lines with slopes m₁ and m₂ satisfies
Atan θ = m₁ · m₂
Btan θ = |m₁ − m₂| / (1 + m₁ m₂)
Ctan θ = m₁ + m₂
Dtan θ = (m₁ m₂) / (m₁ − m₂)
Answer & Solution
Correct answer: B. tan θ = |m₁ − m₂| / (1 + m₁ m₂)
tan θ = |(m₂ − m₁)/(1 + m₁m₂)|. Undefined when m₁m₂ = −1 (lines perpendicular, θ = 90°).
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