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

A. 17 / 30

Step 1: the angle between planes is the angle between their normal vectors. Step 2: normals are (1,5,2) and (2,1,5); dot=17 and both magnitude squares are 30, so cosθ=17/30. 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 (1,5,2) और (2,1,5) हैं; dot=17 और दोनों magnitude square 30 है, इसलिए cosθ=17/30। Step 3: tip: plane angle में coefficients को normal vector मानें।