Q28. For a given series LCR circuit it is found that maximum current is drawn when value of variable capacitance is 25 nF . If resistance of $200 \Omega$ and 100 mH inductor is being used in the given circuit. The frequency of ac source is $\_\_\_\_$ $\times 10 ^ { 3 } \mathrm {~Hz}$. (given $\pi ^ { 2 } = 10$ )