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

C. 8 / 9

Step 1: the angle between planes is the angle between their normal vectors. Step 2: normals are (1,2,2) and (2,1,2); dot=8 and both magnitude squares are 9, so cosθ=8/9. 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 (1,2,2) और (2,1,2) हैं; dot=8 और दोनों magnitude square 9 है, इसलिए cosθ=8/9। Step 3: tip: plane angle में coefficients को normal vector मानें।