Step-by-step explanation: Correct option is C 3 π Normal vector of plane x+y+2z=6 is a = i ^ + j ^ +2 k ^ and normal vector to the plane 2x−y+z=9 is b =2 i ^ − j ^ + k ^ Note: Angle between the plane is same as angle between their normals Thus angle between above planes is =cos −1 ∣ a ∣∣ b ∣ a ⋅ b =cos −1 6 6 2−1+2 =cos −1 2 1 = 3 π Reply

Step-by-step explanation:Correct option is

C

3

π

Normal vector of plane x+y+2z=6 is

a

=

i

^

+

j

^

+2

k

^

and normal vector to the plane 2x−y+z=9 is

b

=2

i

^

−

j

^

+

k

^

Note: Angle between the plane is same as angle between their normals

Thus angle between above planes is =cos

−1

∣

a

∣∣

b

∣

a

⋅

b

=cos

−1

6

6

2−1+2

=cos

−1

2

1

=

3

π