રેખાઓ $a_1x + b_1y + c_1 = 0$ અને $a_2x + b_2y + c_2 = 0$ વચ્ચેનો ખૂણો $\theta$ નીચે મુજબ છે:

• A
  $\tan^{-1} \left| \frac{a_1b_2 - a_2b_1}{a_1a_2 + b_1b_2} \right|$
• B
  $\tan^{-1} \left| \frac{a_1b_2 - a_2b_1}{a_1a_2 + b_1b_2} \right|$
• C
  $\cot^{-1} \left| \frac{a_1a_2 + b_1b_2}{a_1b_2 - a_2b_1} \right|$
• D
  $\tan^{-1} \left| \frac{a_1b_1 - a_2b_2}{a_1a_2 + b_1b_2} \right|$