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NCERT Exemplar · Q90

Q.The order and degree of the differential equation [1+(dydx)2]=d2ydx2\left[1+\left(\frac{dy}{dx}\right)^2\right]=\frac{d^2y}{dx^2} are:
(A) 2, 322,\ \frac{3}{2}
(B) 2, 3
(C) 2, 1
(D) 3, 4

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The order is the highest derivative present (2), and the degree is the power of that highest derivative after removing radicals and fractions — here the equation is already polynomial in d2ydx2\frac{d^2y}{dx^2} with exponent 1, so degree is 1. The correct option is (C).

The order of a differential equation is simply the highest derivative that appears. The degree is trickier: it’s the power of the highest derivative after the equation has been made polynomial in all derivatives — meaning you must clear any radicals, fractions, or fractional powers that involve the derivatives.

Here the equation is:

1+(dydx)2=d2ydx21 + \left(\frac{dy}{dx}\right)^2 = \frac{d^2y}{dx^2}

Let’s walk through it.

  1. Identify the highest derivative.

    The left side has dydx\frac{dy}{dx} (first derivative) and a constant. The right side has d2ydx2\frac{d^2y}{dx^2} (second derivative). No derivative higher than the second appears, so the order is 2.

  2. Check if the equation is already polynomial in the derivatives.

    The term (dydx)2\left(\frac{dy}{dx}\right)^2 is a polynomial term (power 2). The term d2ydx2\frac{d^2y}{dx^2} appears with exponent 1. There are no square roots, no fractional powers, no denominators containing derivatives. The equation is already in the form:

d2ydx2−(dydx)2−1=0\frac{d^2y}{dx^2} - \left(\frac{dy}{dx}\right)^2 - 1 = 0

which is a polynomial in dydx\frac{dy}{dx} and d2ydx2\frac{d^2y}{dx^2}.

  1. Read off the degree. …

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