If we have a conical pendulum, and we are talking about angular momentum and torque about the point of suspension, their magnitudes are constant but their directions are continuously changing. If $\vec{L}$ be the angular momentum then
$$ \frac{\vec{dL}}{dt} = \vec{\tau}$$
Taking the magnitude on both sides, and integrating (since magnitude of torque $\tau$ is constant), we get
$$|\triangle L| = |\tau| \times \triangle t$$
Is this a valid result? I'm getting the wrong answer in a problem and I have a feeling that this is where I went wrong.