According to the well-known method of direct separation of slow and fast motions [1] and [2] an approximate solution of Eq. (1) can be found in the form x=X+Ψ, where X is a slow variable and Ψ is a fast variable. In general, X is regarded as the slow motion with frequency ω and Ψ with frequency Ω. Here, we consider that X, as a slow variable, has many other low-frequencies besides the frequency ω. No matter how many frequency components X contains, as long as X is much slower than Ψ , this method still can be used. Substituting x=X+Ψ into Eq. (1), one obtains