What is the angle between plane 2x-y+z-7=0 and plane x+y+2z- 1 1=0?

Hello, everyone, I'm Miss Zhao from Lu Yingcai Education Center, and the following is my answer:

Analysis: The angle between two planes is the angle between the normal vectors of two planes.

Let a = (2,-1, 1) b = (1, 1, 2),

Then cos (ab) = ab/(| a || b |) = 3/6 =1/2.

So the included angle is 60 degrees.

Hope to adopt, thank you!