(b) Given differential equation is $\frac{d^{2}y}{dx^{2}} = \sqrt{1 + \left( \frac{dy}{dx} \right)^{2}}$ $\Rightarrow$ $\left( \frac{d^{2}y}{dx^{2}} \right)^{2} = 1 + \left( \frac{dy}{dx} \right)^{2}$ Hence, order is 2.
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