Q1. If r⃗ = (cos nt) a⃗ + (sin nt) b⃗, then dr⃗/dt × r⃗ =
Explanation
Differentiate r⃗ to get dr⃗/dt = -n sin nt a⃗ + n cos nt b⃗. Then (dr⃗/dt) × r⃗ = -n sin²nt(a⃗ × b⃗) + n cos²nt(b⃗ × a⃗). Since b⃗ × a⃗ = -(a⃗ × b⃗), this simplifies to n(b⃗ × a⃗), so option A is correct.
