Part of CALC-07 — Differential Equations

Common Traps in Differential Equations

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Trap 1: Degree Not Defined If the DE contains sindydx\frac{dy}{dx}, e^dydx\frac{dy}{dx}, ln(d2yd^{2y}/dx2dx^2), etc., the degree is NOT defined. Only polynomial expressions in derivatives have a degree.

Trap 2: Removing Radicals Changes Degree d2yd^{2y}/dx2dx^2 = sqrt(1 + dydx\frac{dy}{dx}^2) has undefined degree as written. But squaring: (y'')^2 = 1 + (y')^2 gives degree 2. Always simplify before finding degree.

Trap 3: Confusing Order with Degree In (d2yd^{2y}/dx2dx^2)^3 + dy/dx = 0: order = 2 (from d2yd^{2y}/dx2dx^2), degree = 3 (power of highest derivative). These are independent concepts.

Trap 4: Linear vs Bernoulli Misidentification dy/dx + y = y2y^2 is Bernoulli (n=2), NOT linear. Linear requires the right side to be Q(x) only, not Q(x)*yny^n.

Trap 5: Wrong Integrating Factor For dy/dx + Py = Q, IF = e^(integral P dx). Common error: using P = coefficient of y without first dividing the entire equation to make the coefficient of dy/dx equal to 1. Example: x*dy/dx + 2y = x3x^3 must be rewritten as dy/dx + 2y/x = x2x^2 before finding IF.

Trap 6: Homogeneity Check x+y+1(xy+2)\frac{x + y + 1}{(x - y + 2)} is NOT homogeneous because of the constant terms +1 and +2. Translate the origin first to remove constants.

Trap 7: Missing Absolute Value in Separation When separating variables and integrating 1/y, write ln|y| not ln(y). The absolute value matters when the function can be negative.

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