Part of CALC-07 — Differential Equations

Order and Degree — Subtle Distinctions

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The order of a differential equation is the order of the highest derivative present. The degree is the exponent of the highest order derivative, BUT only when the equation is expressed as a polynomial in the derivatives. If the equation involves sindydx\frac{dy}{dx}, e^dydx\frac{dy}{dx}, ln(d2yd^{2y}/dx2dx^2), or fractional powers of derivative expressions, the degree is NOT DEFINED. Common trap: d2yd^{2y}/dx2dx^2 = sqrt(1 + dydx\frac{dy}{dx}^2). This has order 2 but degree is not defined in this form. After squaring: (d2yd^{2y}/dx2dx^2)^2 = 1 + dydx\frac{dy}{dx}^2, the degree becomes 2. JEE frequently tests this distinction. Another example: sindydx\frac{dy}{dx} = x has order 1 but undefined degree because sin is not a polynomial operation on dy/dx.

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