Q.Form the differential equation by eliminating $A$ and $B$ in $Ax^2+By^2=1$.

Showing the 12 most recent of 21 on this concept.

1. AP EAPCET 2024Set eng-2024-05-23-FN1 markMCQ
   Q.The differential equation for which $ax + by =1$ is general solution is (A) $\dfrac{dy}{dx}=x+c$ (B) $y\dfrac{d^2y}{dx^2}+x=1$ (C) $\dfrac{d^2y}{dx^2}=0$ (D) $\dfrac{d^3y}{dx^3}=0$

   ›Reveal solutionSolution

   Two arbitrary constants means two differentiations to eliminate them; doing so on $ax+by=1$ leaves the trivial equation $y''=0$.

   Concept and Intuition

   To find the differential equation whose general solution is a given family, differentiate the family's equation as many times as there are arbitrary constants, then eliminate those constants using the resulting equations. Here $a,b$ are 2 independent constants, so 2 differentiations (and eliminating $a$ and $b$) should produce a second-order DE free of both constants.

   Step-by-Step Solution

   1. Start with $ax+by=1$.
   2. Differentiate w.r.t. $x$: $a+by'=0$ — (i)
   3. Differentiate (i) again w.r.t. $x$: since $a$ is a constant, $\dfrac{d}{dx}(a)=0$, so $by''=0$.
   4. Since $b$ is (generically) nonzero for a genuine two-parameter family, this forces $y''=0$, i.e. $\dfrac{d^2y}{dx^2}=0$. …

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2. AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQ
   Q.If $a$ and $h$ are arbitrary constants, then the differential equation corresponding to the family of curves $ax^2 + 2hxy = 1$ is (A) $x^2\dfrac{d^2y}{dx^2} + x\dfrac{dy}{dx} + y = 0$ (B) $x^2\dfrac{d^2y}{dx^2} - x\dfrac{dy}{dx} + y = 0$ (C) $x^2\dfrac{d^2y}{dx^2} + x\dfrac{dy}{dx} - y = 0$ (D) $x^2\dfrac{d^2y}{dx^2} - x\dfrac{dy}{dx} - y = 0$

   ›Reveal solutionSolution

   With two arbitrary constants $a,h$, we differentiate the family twice and eliminate both constants between the resulting equations, landing on $x^2y''+xy'-y=0$.

   Concept and Intuition

   A family of curves with $n$ independent arbitrary constants satisfies a differential equation of order $n$ (generically), obtained by differentiating the family $n$ times and eliminating the constants among the original equation and its derivatives. Here there are two constants ($a$ and $h$), so we expect (and need) a second-order ODE.

   Step-by-Step Solution

   1. Start with $ax^2 + 2hxy = 1$.
   2. Differentiate once w.r.t. $x$:

   $2ax + 2h(xy'+y) = 0 \;\Longrightarrow\; ax + h(xy'+y) = 0 \quad \text{...(I)}$

   3. Differentiate (I) again w.r.t. $x$:

   $a + h\big(xy''+y'+y'\big) = 0 \;\Longrightarrow\; a + h(xy''+2y') = 0 \quad \text{...(II)}$

   4. From (I): $a = -\dfrac{h(xy'+y)}{x}$ (for $x\neq0$).
   5. Substitute into (II):

   $-\frac{h(xy'+y)}{x} + h(xy''+2y') = 0$

   6. Divide through by $h$ (assuming $h\neq0$) and multiply by $x$:

   $-(xy'+y) + x^2y''+2xy' = 0$

   7. Simplify: …

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3. AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQ
   Q.The differential equation obtained by eliminating $A$ and $B$ from the equation $y = A(x+B)^2$ which represents a family of curves (A) $2yy'' = (y')^2$ (B) $yy'' = 2y'$ (C) $2yy'' = y' + y$ (D) $2yy'' = y' - y$

   ›Reveal solutionSolution

   Differentiating the two-parameter family $y=A(x+B)^2$ twice and eliminating $A,B$ algebraically yields the differential equation $2yy''=(y')^2$.

   Concept and Intuition

   A family of curves with $n$ arbitrary constants generally satisfies an $n$th-order differential equation obtained by differentiating $n$ times and eliminating the constants. Here there are two constants ($A$ and $B$), so we differentiate twice and use the original equation plus its first and second derivatives to eliminate both.

   Step-by-Step Solution

   1. Given $y=A(x+B)^2$.
   2. Differentiate once: $y'=2A(x+B)$.
   3. Differentiate again: $y''=2A$, so $A=\dfrac{y''}{2}$.
   4. From step 2, solve for $(x+B)$: $(x+B)=\dfrac{y'}{2A}=\dfrac{y'}{y''}$ (substituting $2A=y''$). …

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4. AP EAPCET 2025Set eng-2025-05-21-FN1 markMCQ
   Q.The differential equation corresponding to the family of parabolas whose axis is along $x = 1$ is (A) $\dfrac{d^2y}{dx^2}-(x-1)\dfrac{dy}{dx}=0$ (B) $(x-1)\dfrac{d^2y}{dx^2}-\dfrac{dy}{dx}=0$ (C) $\dfrac{d^2y}{dx^2}+(x-1)\dfrac{dy}{dx}-y=0$ (D) $(x-1)\dfrac{d^2y}{dx^2}+\dfrac{dy}{dx}=0$

   ›Reveal solutionSolution

   This tests forming a differential equation by eliminating arbitrary constants from a family of curves; the answer is (B).

   Concept and Intuition

   A family of parabolas with a fixed vertical axis $x=1$ has the general equation $y=a(x-1)^2+b$, where $a$ (controls width/orientation) and $b$ (vertical shift of vertex) are the two free parameters. Since there are 2 arbitrary constants, we need a 2nd-order ODE to eliminate them completely.

   Step-by-Step Solution

   1. Write the family: $y=a(x-1)^2+b$.
   2. Differentiate once: $\dfrac{dy}{dx}=2a(x-1)$.
   3. Differentiate again: $\dfrac{d^2y}{dx^2}=2a$, so $a=\dfrac12\dfrac{d^2y}{dx^2}$.
   4. Substitute this back into the first derivative relation: $\dfrac{dy}{dx}=\left(\dfrac{d^2y}{dx^2}\right)(x-1)$.
   5. Rearranging: $(x-1)\dfrac{d^2y}{dx^2}-\dfrac{dy}{dx}=0$, which eliminates both $a$ and $b$ (note $b$ already dropped out automatically after the first derivative). …

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5. AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQ
   Q.If a and b are arbitrary constants, then the differential equation corresponding to the family of curves $y = \tan(ax+b)$ is (A) $(1+x^2)y_2 - 2yy_1 + y = 0$ (B) $(1+y^2)y_2 - 2yy_1^2 = 0$ (C) $(1+x^2)y_2 + 2yy_1^2 = 0$ (D) $(1+y^2)y_2 - 2yy_1^2 + y = 0$

   ›Reveal solutionSolution

   Differentiating $y=\tan(ax+b)$ twice and eliminating the arbitrary constant $a$ (using $\sec^2=1+\tan^2$) directly gives the second-order ODE $(1+y^2)y_2-2yy_1^2=0$.

   Concept and Intuition

   A 2-parameter family of curves satisfies a second-order differential equation obtained by differentiating twice and eliminating both arbitrary constants. Since $b$ only shifts the argument (it never appears explicitly after one differentiation of $\tan$), only $a$ needs eliminating here.

   Step-by-Step Solution

   1. $y=\tan(ax+b)$. First derivative: $y_1=a\sec^2(ax+b)=a(1+\tan^2(ax+b))=a(1+y^2)$.
   2. So $a=\dfrac{y_1}{1+y^2}$ — note $b$ has already dropped out.
   3. Differentiate $y_1=a(1+y^2)$ again with respect to $x$: $y_2=a\cdot2yy_1$ (since $a$ is constant).
   4. Substitute $a=\dfrac{y_1}{1+y^2}$: $y_2=\dfrac{y_1}{1+y^2}\cdot2yy_1=\dfrac{2yy_1^2}{1+y^2}$. …

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6. AP EAPCET 2024Set eng-2024-05-22-AN1 markMCQ
   Q.The differential equation of the family of hyperbolas having their centres at origin and their axes along the coordinates axes is (A) $xyy_2+xy_1^2-yy_1=0$ (B) $xy_2-xyy_1^2+yy_1=0$ (C) $xyy_2+xy_1^2+yy_1=0$ (D) $xy_2+xy_1^2-yy_1=0$

   ›Reveal solutionSolution

   Differentiating the two-parameter hyperbola family $\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$ twice and eliminating $a^2,b^2$ gives $xyy_2+xy_1^2-yy_1=0$, option (A).

   Concept and Intuition

   A family of curves with $n$ independent arbitrary constants satisfies an $n$-th order differential equation obtained by differentiating $n$ times and eliminating the constants. Here the hyperbola family centred at the origin with axes along the coordinate axes has two constants $a,b$, so we need two differentiations.

   Step-by-Step Solution

   1. Family: $\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1$.
   2. Differentiate once: $\dfrac{2x}{a^2}-\dfrac{2yy_1}{b^2}=0\Rightarrow\dfrac{x}{a^2}=\dfrac{yy_1}{b^2}\Rightarrow\dfrac{a^2}{b^2}=\dfrac{x}{yy_1}$.
   3. Differentiate again: $\dfrac{1}{a^2}-\dfrac{(y_1^2+yy_2)}{b^2}=0\Rightarrow\dfrac{1}{a^2}=\dfrac{y_1^2+yy_2}{b^2}$.
   4. Divide the two boxed relations (both equal to a ratio of $1/a^2$ terms with $1/b^2$ factored) to eliminate $a^2,b^2$ entirely:

   $x=\frac{a^2\cdot\frac{yy_1}{b^2}\Big/\frac{1}{a^2}}{\cdots}\ \Rightarrow\ x=\frac{yy_1}{y_1^2+yy_2}$

   (dividing the step-2 relation by the step-3 relation directly cancels $a^2/b^2$). …

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7. AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQ
   Q.The differential equation for which $y^2=4a(x+a)$ (a is the parameter) is the general solution is (A) $y=2x\dfrac{dy}{dx}+y\left(\dfrac{dy}{dx}\right)^2$ (B) $y=y\dfrac{dy}{dx}-x\left(\dfrac{dy}{dx}\right)^2$ (C) $x=3\dfrac{dy}{dx}+y\left(\dfrac{dy}{dx}\right)^2$ (D) $y=3x^2\dfrac{dy}{dx}+y^2\left(\dfrac{dy}{dx}\right)^2$

   ›Reveal solutionSolution

   This tests eliminating the arbitrary constant from a one-parameter family to get its differential equation. Differentiate once, solve for $a$, substitute back. Answer: (A).

   Concept and Intuition

   A family of curves with $n$ independent parameters satisfies a differential equation of order $n$. Here only $a$ is a parameter, so one differentiation should let us eliminate it completely and land back on a relation purely in $x, y, y'$.

   Step-by-Step Solution

   1. Start with $y^2 = 4a(x+a) = 4ax + 4a^2$.
   2. Differentiate both sides with respect to $x$ (treating $a$ as constant):

   $2y\frac{dy}{dx} = 4a \implies a = \frac{1}{2}y\frac{dy}{dx}.$

   3. Substitute this expression for $a$ back into the original equation to eliminate $a$:

   $y^2 = 4x\left(\frac{1}{2}yy'\right) + 4\left(\frac{1}{2}yy'\right)^2 = 2xyy' + y^2(y')^2.$

   4. Divide throughout by $y$ (valid away from $y=0$):

   $y = 2x\frac{dy}{dx} + y\left(\frac{dy}{dx}\right)^2.$

   5. This matches option (A) exactly. …

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8. AP EAPCET 2024Set eng-2024-05-21-FN1 markMCQ
   Q.The differential equation formed by eliminating $a$ and $b$ from the equation $y = ae^{2x} + bxe^{2x}$ is (A) $y'' - 4y' - 4y = 0$ (B) $y'' + 4y' - 4y = 0$ (C) $y'' + 4y' + 4y = 0$ (D) $y'' - 4y' + 4y = 0$

   ›Reveal solutionSolution

   The form $y=(a+bx)e^{2x}$ is the textbook general solution for a repeated characteristic root $r=2$, so the corresponding differential equation is read off directly from $(r-2)^2=0$.

   Concept and Intuition

   Whenever a family of functions has the shape $(a+bx)e^{rx}$, it is exactly the general solution of a second-order linear ODE with constant coefficients whose characteristic equation has a repeated root at $r$. Recognizing this shape avoids needing to differentiate twice and eliminate constants by hand.

   Step-by-Step Solution

   1. $y=ae^{2x}+bxe^{2x}=(a+bx)e^{2x}$ — this is the standard form for a repeated root $r=2$ of the auxiliary equation.
   2. The auxiliary equation with repeated root $2$ is $(r-2)^2=0 \Rightarrow r^2-4r+4=0$.
   3. This corresponds to the ODE $y''-4y'+4y=0$. …

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9. AP EAPCET 2024Set eng-2024-05-18-FN1 markMCQ
   Q.The differential equation representing the family of circles having their centres on Y-axis is $\left(y_1=\dfrac{dy}{dx} \text{ and } y_2=\dfrac{d^2y}{dx^2}\right)$ (A) $y_2=y(y_1^2+1)$ (B) $y_2=xy(y_1^2+1)$ (C) $xy_2=y_1(y_1^2+1)$ (D) $xy_2=y(y_1^2+1)$

   ›Reveal solutionSolution

   Eliminating the two constants $k$ (center) and $a$ (radius) from $x^2+(y-k)^2=a^2$ via two differentiations yields $xy_2=y_1(y_1^2+1)$.

   Concept and Intuition

   A family of curves with $n$ independent arbitrary constants satisfies a differential equation of order $n$ obtained by differentiating $n$ times and eliminating the constants. Circles centered anywhere on the Y-axis have two free parameters — the center's y-coordinate $k$ and the radius $a$ — so we need exactly two differentiations.

   Step-by-Step Solution

   1. General equation of a circle with center $(0,k)$ and radius $a$: $x^2+(y-k)^2=a^2$.
   2. Differentiate once w.r.t. $x$: $2x+2(y-k)y_1=0\Rightarrow x+(y-k)y_1=0\Rightarrow (y-k)=-\dfrac{x}{y_1}$.
   3. Differentiate again: $1+y_1\cdot y_1+(y-k)y_2=0$, i.e. $1+y_1^2+(y-k)y_2=0$.
   4. Substitute $(y-k)=-x/y_1$ from step 2: $1+y_1^2-\dfrac{x}{y_1}y_2=0$. …

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10. AP EAPCET 2022Set eng-2022-07-08-FN1 markMCQ
    Q.If the differential equation obtained by eliminating A, B from $y = (\sin^{-1}x)^2 + A\cos^{-1}x + B$ is $(a - x^2)y'' - xy' = b$, then $\dfrac{b+a}{b-a} =$ (A) $2$ (B) $-2$ (C) $3$ (D) $-3$

    ›Reveal solutionSolution

    Differentiating the given function twice eliminates both arbitrary constants and produces $(1-x^2)y''-xy'=2$; matching gives $a=1,b=2$ and $\frac{b+a}{b-a}=3$.

    Concept and Intuition

    A function with two arbitrary constants ($A,B$) satisfies a second-order differential equation obtained by differentiating twice (removing both constants). We just need to carry out that differentiation carefully, using $\dfrac{d}{dx}\sin^{-1}x = \dfrac{1}{\sqrt{1-x^2}}$ and $\dfrac{d}{dx}\cos^{-1}x = \dfrac{-1}{\sqrt{1-x^2}}$.

    Step-by-Step Solution

    1. $y = (\sin^{-1}x)^2 + A\cos^{-1}x + B$.
    2. First derivative: $y' = 2\sin^{-1}x\cdot\dfrac{1}{\sqrt{1-x^2}} - \dfrac{A}{\sqrt{1-x^2}} = \dfrac{2\sin^{-1}x - A}{\sqrt{1-x^2}}$.
    3. Rearrange: $y'\sqrt{1-x^2} = 2\sin^{-1}x - A$ — this has eliminated $B$ and still contains $A$ (as an additive constant), but note the combination is now clean.
    4. Differentiate again: $y''\sqrt{1-x^2} + y'\cdot\dfrac{-x}{\sqrt{1-x^2}} = \dfrac{2}{\sqrt{1-x^2}}$. …

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11. AP EAPCET 2024Set eng-2024-05-21-AN1 markMCQ
    Q.The differential equation formed by eliminating arbitrary constants $A, B$ from the equation $y = A\cos 3x + B\sin 3x$ is (A) $\dfrac{d^2y}{dx^2} + y = 0$ (B) $\dfrac{d^2y}{dx^2} + 9y = 0$ (C) $\dfrac{d^2y}{dx^2} - 9y = 0$ (D) $\dfrac{d^2y}{dx^2} - y = 0$

    ›Reveal solutionSolution

    Since $y=A\cos3x+B\sin3x$, differentiating twice reproduces $-9y$, giving $y''+9y=0$.

    Concept and Intuition

    For $y=A\cos(kx)+B\sin(kx)$, each differentiation brings down a factor of $k$ and eventually $y''=-k^2y$ — the signature ODE of simple harmonic motion with angular frequency $k$.

    Step-by-Step Solution

    1. $y=A\cos3x+B\sin3x$.
    2. $y'=-3A\sin3x+3B\cos3x$.
    3. $y''=-9A\cos3x-9B\sin3x=-9(A\cos3x+B\sin3x)=-9y$.
    4. So $y''+9y=0$. …

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12. AP EAPCET 2022Set eng-2022-07-07-FN1 markMCQ
    Q.The differential equation of the family of circles passing through (0, 0) and having centre on x-axis is (A) $2xy\dfrac{dy}{dx} + x^2 - y^2 = 0$ (B) $\left(\dfrac{dy}{dx}\right)^2 + y\dfrac{d^2y}{dx^2} + 1 = 0$ (C) $xy\dfrac{dy}{dx} + y^2 - x^2 = 0$ (D) $\dfrac{dy}{dx} = \dfrac{x+y}{x-y}$

    ›Reveal solutionSolution

    Eliminating the one-parameter family's constant $h$ (the centre's x-coordinate) between the circle equation and its derivative gives $2xyy'+x^2-y^2=0$.

    Concept and Intuition

    A family of circles through the origin with centres on the x-axis is a one-parameter family (parameter $h$, the centre). To get its differential equation, differentiate the family once (matching the one free parameter) and eliminate $h$ between the original equation and the derived one.

    Step-by-Step Solution

    1. Circle centred at $(h,0)$ through origin: radius $=$ distance from centre to origin $=h$. Equation: $(x-h)^2+y^2=h^2\Rightarrow x^2+y^2-2hx=0$.
    2. Differentiate w.r.t. $x$: $2x+2yy'-2h=0\Rightarrow h=x+yy'$.
    3. From the original equation, $h=\dfrac{x^2+y^2}{2x}$.
    4. Equate: $x+yy'=\dfrac{x^2+y^2}{2x}$. Multiply by $2x$: $2x^2+2xyy'=x^2+y^2$. …

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