Q.(vi) The differential equation representing the family of circles x2+(y−a)2=a2 will be of order two. (State True or False.)
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Order of a Differential Equation
A differential equation involves an unknown function together with its derivatives dxdy,dx2d2y,dx3d3y,…. The order of the equation is simply the order of the highest derivative that appears in it.
So to find the order, scan the equation, find the most-differentiated term, and read off how many times y has been differentiated there.
Some examples
- dxdy+3y=0 — the highest derivative is the first derivative, so the order is 1.
- dx2d2y+5(dxdy)3+y=0 — the highest derivative present is dx2d2y, so the order is 2. (The cube on dxdy is a power, not a higher order.)
- (dx3d3y)2+dx2d2y=sinx — the highest is the third derivative, so the order is 3.
Do not confuse order with degree. Order = the order of the highest derivative present. Degree = the power of that highest-order derivative once the equation is written free of radicals and fractions in the derivatives. Raising a derivative to a power changes the degree, never the order.
Why order matters …
The family x2+(y−a)2=a2 has just one arbitrary constant a. The order of the differential equation representing a family equals the number of independent arbitrary constants, so eliminating a needs only one differentiation and gives a first-order equation. …
The family has only one arbitrary constant, so its differential equation has order one. The statement is False.
The key principle
The order of the differential equation that represents a family of curves equals the number of independent arbitrary constants in the family, because each constant needs one differentiation to eliminate it.
The family
x2+(y−a)2=a2
contains a single arbitrary constant a (it fixes both the centre (0,a) and the radius a at once). So we expect order one. Let us confirm it.
Eliminate the constant
1. Simplify. Expand:
x2+y2−2ay+a2=a2 ⇒ x2+y2=2ay.
2. Differentiate once with respect to x:
2x+2ydxdy=2adxdy.
3. Remove a. From step 1, a=2yx2+y2. Substituting,
x+ydxdy=2yx2+y2dxdy.
Multiply by 2y and collect the derivative terms: …
Method: Order of the DE Representing a Family of Curves
The order of the differential equation that represents a family equals the number of independent arbitrary constants in the family — count them first, before differentiating.
Steps
Step 1: Count the essential arbitrary constants.
Look at the family and identify how many independent parameters it truly contains (not how many times a letter appears).
Step 2: Differentiate just enough times.
To eliminate n constants you differentiate n times; the resulting equation has order n.
Step 3: Eliminate the constant(s). …
Common Mistakes
Mistake 1: Counting a and a2 as two different constants.
Why it's wrong: x2+(y−a)2=a2 contains a single parameter a; a2 is not independent of a. Correct approach: count independent arbitrary constants — here just one — so the order is one.
Mistake 2: Assuming a "circle" family must give a second-order equation. …
Showing the 12 most recent of 13 on this concept.
- GUJCET 2021Set 151 markMCQQ.Order and degree of the differential equation edx2d2y=x are respectively. (A) 2 and not defined (B) 1 and 2 (C) 2 and 1 (D) 1 and not defined
›Reveal solutionSolution
[!TLDR]
Order =2, degree = not defined.
Concept
- Order = order of the highest derivative appearing.
- Degree = the power of the highest-order derivative, but only after the equation is expressed as a polynomial in all the derivatives. If the derivative appears inside a transcendental function (e(⋅), sin, log, etc.), no such polynomial form exists and the degree is undefined.
Solution …
- GUJCET 2020Set 071 markMCQQ.The order and degree of differential equation {1+(dxdy)2}3/2=dx2d2y are p and q respectively then p+q= ________. (A) 6 (B) 4 (C) 2 (D) 5
›Reveal solutionSolution
Highest derivative is y′′ (order 2); after squaring to remove the 3/2 power, its power is 2 (degree 2), so p+q=4.
Concept: Order = highest derivative present; degree = power of that highest derivative once the equation is a polynomial in derivatives.
{1+(dxdy)2}3/2=dx2d2y.
Square both sides to clear the 3/2 power: …
- GUJCET 2024Set 131 markMCQQ.The order and the degree of the differential equation dx2d2y=3(dxdy)4+2 is respectively __________ and __________. (A) 1,8 (B) 3,2 (C) 2,8 (D) 2,3
›Reveal solutionSolution
Order = 2 (the dx2d2y term); after removing the fractional powers the degree of that highest derivative is 3.
Order. The highest derivative present is dx2d2y, so order =2. …
- GUJCET 2025Set 031 markMCQQ.The degree of the differential equation (1+dxdy)21=(dx2d2y)31 is _____ (A) 4 (B) 2 (C) 3 (D) 1
›Reveal solutionSolution
Make the equation polynomial in derivatives, then read the power of the highest-order derivative.
Raise both sides of (1+dxdy)1/2=(dx2d2y)1/3 to the 6th power: …
- GUJCET 2023Set 091 markMCQQ.The order and degree of the differential equation 4(dx3d3y)5=3(dx2d2y)4 is : (A) 2 and 16 (B) 3 and 15 (C) 3 and 16 (D) 2 and 12
›Reveal solutionSolution
Order is the highest derivative; degree is its power after removing all radicals.
Concept. 4(y′′′)5=3(y′′)4 means (y′′′)5/4=(y′′)4/3. Highest derivative is y′′′, so order =3.
Solution. Raise both sides to the 12th power to clear fractional exponents: …
- GUJCET 2019Set 171 markMCQQ.If the general solution of some differential equation is y=a1(a2+a3)⋅cos(x+a4)−a5ex+a6 then order of differential equation is . (A) 5 (B) 4 (C) 6 (D) 3
›Reveal solutionSolution
Count the independent arbitrary constants.
Concept. The number of independent arbitrary constants equals the order of the differential equation.
Steps.
- a1(a2+a3)cos(x+a4): the product a1(a2+a3) is one constant and a4 another; but Ccos(x+a4)=Acosx+Bsinx gives 2 independent constants. …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.The degree of the differential equation (1+y12)3/2=y2 is ____.(a) 4(b) 3(c) 2(d) Not defined
›Reveal solutionSolution
Clear the fractional power by squaring, then read off the power of the highest-order derivative.
(1+y12)3/2=y2. Squaring both sides to remove the fractional exponent: (1+y12)3=y22.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.The order of the differential equation (dx2d2y)3+(dxdy)2+cos(dxdy)+1=0 = ____.(a) 3(b) 2(c) 1(d) Not defined
›Reveal solutionSolution
Order is simply the highest derivative appearing, regardless of the (undefined) degree caused by the cos term.
The equation contains dx2d2y (raised to a power, inside no transcendental function) and dxdy (which appears inside cos(⋅), making the degree undefined -- but order is unaf …
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.The order of the differential equation (dx3d3y)4+(dx2d2y)2+sin(dxdy)+1=0 is ______.(a) 3(b) 4(c) 2(d) undefined
›Reveal solutionSolution
The order of a differential equation is the order of the highest derivative present, regardless of the power it's raised to or functions applied to lower-order terms.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2022Set ANNUAL1 markMCQQ.The order and degree of the differential equation 1+(dxdy)2=dx2d2y respectively are ___.(a) 1,2(b) 2,2(c) 2,1(d) 4,2
›Reveal solutionSolution
Rationalise, then read off the order (highest derivative) and degree (its power).
1+(dxdy)2=dx2d2y.
Squaring: (1+(dxdy)2)2=dx2d2y.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2020Set ANNUAL1 markMCQQ.The order and degree of the differential equation (y′′′)3+(y′′)4+(y′)4+y=7 are ___ respectively.(a) 3 and 3(b) 1 and 4(c) 4 and 1(d) 2 and 4
›Reveal solutionSolution
Order = order of the highest derivative appearing; degree = the power of that highest-order derivative (once the equation is a polynomial in derivatives).
(y′′′)3+(y′′)4+(y′)4+y=7. The highest-order derivative present is y′′′ (third derivative), so order =3.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2019Set ANNUAL1 markMCQQ.The order and degree of dx2d2y=31+(dxdy)2 are ______ respectively.(a) 2,3(b) 3,2(c) 3, not defined(d) 2,2
›Reveal solutionSolution
Order = order of the highest derivative present; degree = its power once the equation is a polynomial in derivatives.
The highest derivative is dx2d2y, so the order is 2. To find the degree, remove the fractional power by cubing both sides: (dx2d2y)3=1+(dxdy)2. Now the equation is a polynomi …
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