For planes x+6y+2z=5 and 2x+y+6z=7, what is cos theta between them?

C. 20 / 41

Step 1: the angle between planes is the angle between their normal vectors. Step 2: normals are (1,6,2) and (2,1,6); dot=20 and both magnitude squares are 41, so cosθ=20/41. Step 3: tip: use plane coefficients as normal vectors.

Step 1: planes का angle उनके normal vectors के angle जैसा होता है। Step 2: normals (1,6,2) और (2,1,6) हैं; dot=20 और दोनों magnitude square 41 है, इसलिए cosθ=20/41। Step 3: tip: plane angle में coefficients को normal vector मानें।