4. The value of \( \left(\frac{1}{2}\right)^{-2}+\left(\frac{1}{3}\right)^{-2}+\left(\frac{1}{4}\right)^{-2} \) is
(a) \( \frac{61}{144} \)
(b) \( \frac{144}{61} \)
(c) 29
(d) none of these
Correct answer is (b)
We know that
\(\tau\) = r × F
∵ \(\tau\) \(= \frac{dL}{dt},\) \(F = \frac{dP}{dt}\)
\(\left(\frac{dL}{dt}\right) = r \times \left(\frac{dP}{dt}\right)\)
\(\left(\frac{dL}{dt}\right) - r \times \left(\frac{dP}{dt}\right) = 0\)