For planes x+8y+2z=5 and 2x+y+8z=7, what is cos theta between them?

C. 26 / 69

Step 1: the angle between planes is the angle between their normal vectors. Step 2: normals are (1,8,2) and (2,1,8); dot=26 and both magnitude squares are 69, so cosθ=26/69. 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 (1,8,2) और (2,1,8) हैं; dot=26 और दोनों magnitude square 69 है, इसलिए cosθ=26/69। Step 3: tip: plane angle में coefficients को normal vector मानें।