1. Determine if the following equations are linear or nonlinear, find theirorder: \text { a. } y^{\prime \prime}+2 y y^{\prime}+x=2 x+1 \text {; } \text { b. } \frac{d^{3} y}{d x^{3}}+x^{4} \frac{d^{2} y}{d x^{2}}=\cos (x) \text {. }

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