The point A, B, C, D have co-ordinates as follows $$\ A: (2,1,-2) ~~~~~~B: (4,1,-1)~~~~ ~C: (3,-2,-1)~~~~~~~ D: (3,6,2) $$
The plane R passes through A,B and C and plane Q passes through A,B and D. Find the acute angle between the two planes.
I am aware of the fool proof method of finding the acute angle between the normal vectors of the plane. However, I tried finding the angle between AC and AD. Since one plane contains the line AC and the second contains the line AD, finding the angle between these lines should provide me the angle between the planes. However, the answer is not coming correct. Can someone point out the flaw in my method?