In Rayleigh's method, which of the following energy at the mean position is equal to the maximum potential energy (or strain energy) at the extreme position?
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Rayleigh's Method summary formula:
\[
\text{K.E.}_{\max} = \text{P.E.}_{\max}
\]
For a lump mass system with displacement \(x = X\sin(\omega_n t)\):
\[
\frac{1}{2} m (\omega_n X)^2 = \frac{1}{2} k X^2 \quad \Rightarrow \quad \omega_n = \sqrt{\frac{k}{m}}
\]