Consider motion in the xy-plane governed by a Hamiltonian H(p_x , p_y, x, y). Introduce polar coordinates x = r cos?, y = r sin ?, p_x = p cos ?_p, p_y = p sin ?_p, and use the chain rule for partial derivatives,e.g. ?H/?? = (?H/?x)(?x/??) + (?H/?y)(?y/??), etc.,to derive eq. (21.17).

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i = {L, H}