Cosin của góc giữa hai đường thẳng Δ1: a1x + b1y + c1 = 0 và Δ2: a2x + b2y + c2 = 0 là

A. $\cos ({∆}_1;{∆}_2)=\frac{\left|a_1{b}_1+a_2{b}_2\right|}{\sqrt{{a_1}^2+{b_1}^2}.\sqrt{{a_2}^2+{b_2}^2}}$

B. $\cos ({∆}_1;{∆}_2)=\frac{\left|a_1{a}_2+b_1{b}_2\right|}{\sqrt{{a_1}^2+{a_2}^2}.\sqrt{{b_1}^2+{b_2}^2}}$

C. $\cos ({∆}_1;{∆}_2)=\frac{\left|a_1{a}_2+b_1{b}_2\right|}{\sqrt{{a_1}^2+{b_1}^2}.\sqrt{{a_2}^2+{b_2}^2}}$

D. $\cos ({∆}_1;{∆}_2)=\frac{a_1{a}_2+b_1{b}_2}{\sqrt{{a_1}^2+{b_1}^2}.\sqrt{{a_2}^2+{b_2}^2}}$