Cho hai mặt phẳng (P): ax +by +cz +d = 0, (Q): a'x +b'y +c'z +d' = 0

A. $\cos \left(\left(P\right);\left(Q\right)\right)=\frac{\left|a.a\text{'}+b.b\text{'}+c.c\text{'}\right|}{\sqrt{a^2+b^2+c^2}.\sqrt{a{\text{'}}^2+b{\text{'}}^2+c{\text{'}}^2}}$

B. $\cos \left(\left(P\right);\left(Q\right)\right)=\frac{a.a\text{'}+b.b\text{'}+c.c\text{'}}{\sqrt{a^2+b^2+c^2}.\sqrt{a{\text{'}}^2+b{\text{'}}^2+c{\text{'}}^2}}$

C. $\cos \left(\left(P\right);\left(Q\right)\right)=\frac{a.a\text{'}+b.b\text{'}+c.c\text{'}}{\sqrt{a+b+c}.\sqrt{a\text{'}+b\text{'}+c\text{'}}}$ 

D. $\cos \left(\left(P\right);\left(Q\right)\right)=\frac{\left|a.a\text{'}+b.b\text{'}+c.c\text{'}\right|}{{\sqrt{a+b+c}}^2.{\sqrt{a\text{'}+b\text{'}+c\text{'}}}^2}$