Q.Solve the following differential equation: ydx+xlog(xy)dy−2xdy=0
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Homogeneous Differential Equation
Homogeneous Differential Equations
The idea: only the ratio y/x matters
A function f(x,y) is homogeneous of degree n if scaling both variables by t scales the function by tn: f(tx,ty)=tnf(x,y). For example f(x,y)=x2+y2 gives f(tx,ty)=t2(x2+y2) — degree 2.
A first-order equation
dxdy=f(x,y)
is called homogeneous when f is homogeneous of degree 0, i.e. f(tx,ty)=f(x,y). Scaling then changes nothing, which means the slope depends only on the ratio y/x, never on the absolute sizes. So a homogeneous equation can always be recast as
dxdy=F(xy).
Quick test in the form M(x,y)dx+N(x,y)dy=0: if every term of M and N has the same total degree (sum of the powers of x and y), the equation is homogeneous. E.g. in (x2+y2)dx−2xydy=0 both M and N are degree 2.
Why we care: it becomes separable
Recognising homogeneity buys you a guaranteed method. Substitute
y=vx⇒dxdy=v+xdxdv.
Putting this into dxdy=F(v) gives
v+xdxdv=F(v)⇒xdxdv=F(v)−v,
which separates:
F(v)−vdv=xdx.
Integrate both sides, then replace v by y/x to return to the original variables. …
Both M=y and N=xlog(y/x)−2x are homogeneous of degree 1, so put y=vx, dy=vdx+xdv, log(y/x)=logv.
vxdx+x(logv−2)(vdx+xdv)=0.
Divide by x and collect dx:
v(logv−1)dx+x(logv−2)dv=0 ⇒ xdx+v(logv−1)logv−2dv=0.
With t=logv (dt=vdv): ∫t−1t−2dt=∫(1−t−11)dt=t−log∣t−1∣. So
log∣x∣+logv−log∣logv−1∣=C. …
The equation is homogeneous; y=vx separates it, and the general solution is y=k[log(y/x)−1].
Why homogeneous
Write it as Mdx+Ndy=0 with
M=y,N=xlog(xy)−2x.
Every term is degree 1 in x,y, so the equation is homogeneous and y=vx will separate it.
Substitute y=vx
With dy=vdx+xdv and log(y/x)=logv,
vxdx+x(logv−2)(vdx+xdv)=0.
Divide by x:
vdx+v(logv−2)dx+x(logv−2)dv=0.
Collect the dx terms: v[1+logv−2]=v(logv−1), so
v(logv−1)dx+x(logv−2)dv=0.
Separate
Divide by xv(logv−1):
xdx+v(logv−1)logv−2dv=0.
Integrate
For the second term put t=logv, dt=vdv:
∫t−1t−2dt=∫(1−t−11)dt=t−log∣t−1∣.
Hence
log∣x∣+logv−log∣logv−1∣=C.
Return to x,y
With v=xy, note log∣x∣+logv=log∣x∣+log∣y∣−log∣x∣=log∣y∣, so …
Method: Homogeneous equation better solved with x=vy
Use this when y appears more simply than x (e.g. logs of y/x with dy terms), as in ydx+xlogxydy−2xdy=0; substituting x=vy can be cleaner than y=vx.
Steps
Step 1: Choose the tidier substitution
Because the equation is homogeneous, either y=vx or x=vy works; pick the one that simplifies the logarithm. With x=vy, dydx=v+ydydv.
Step 2: Substitute and separate
Express everything in v=yx, cancel matching terms, and separate the v and y parts. …
Common Mistakes
Mistake 1: Insisting on y=vx when x=vy is cleaner
Why it's wrong: with logxy and dy terms, y=vx makes messy logs; x=vy simplifies them. Correct approach: pick the substitution that tidies the ratio.
Mistake 2: Wrong derivative form for the chosen substitution …
Showing the 12 most recent of 32 on this concept.
- AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQQ.The general solution of the differential equation y(2x+y)dx=x(x+y)dy is (A) log(ycx2)+xy=0 (B) log(cx2y)+xy=0 (C) log(cx2y2)+xy=0 (D) log(cx2y)+xy=0
›Reveal solutionSolution
A homogeneous ODE solved by the substitution y=vx; simplifying and integrating gives log(cx2y)+xy=0.
Concept and Intuition
y(2x+y)dx=x(x+y)dy is homogeneous of degree 2 on both sides, so the standard substitution y=vx (with dy=vdx+xdv) reduces it to a separable equation in v and x.
Step-by-Step Solution
- Put y=vx, dy=vdx+xdv.
- LHS: y(2x+y)=vx(2x+vx)=vx2(2+v).
- RHS: x(x+y)dy=x⋅x(1+v)(vdx+xdv)=x2(1+v)(vdx+xdv).
- Equation becomes vx2(2+v)dx=x2(1+v)(vdx+xdv). Divide by x2: v(2+v)dx=v(1+v)dx+x(1+v)dv.
- Subtract v(1+v)dx from both sides: v[(2+v)−(1+v)]dx=x(1+v)dv⇒vdx=x(1+v)dv.
- Separate: xdx=v1+vdv=(v1+1)dv.
- Integrate: logx=logv+v+C⇒logvx−v=C.
- Substitute back v=y/x, so x/v=x2/y: logyx2−xy=C. …
- AP EAPCET 2025Set eng-2025-05-21-FN1 markMCQQ.The general solution of the differential equation (xsinxy)dy=(ysinxy−x)dx is (A) logx+tanxy=c (B) logx+cosxy=c (C) logx−sinxy=c (D) logx−cosxy=c
›Reveal solutionSolution
This is a homogeneous first-order ODE solved via y=vx; the answer is (D).
Concept and Intuition
When every term in the ODE is a function of y/x times some power of x or y (a homogeneous equation), the substitution y=vx (so v=y/x) reduces it to a separable equation in v and x.
Step-by-Step Solution
- The given equation is (xsinxy)dy=(ysinxy−x)dx, so
dxdy=xsin(y/x)ysin(y/x)−x.
- Substitute y=vx, dxdy=v+xdxdv. The right side becomes xsinvvxsinv−x=sinvvsinv−1=v−sinv1.
- So v+xdxdv=v−sinv1, giving xdxdv=−sinv1.
- Separate variables: sinvdv=−xdx. …
- AP EAPCET 2024Set eng-2024-05-20-FN1 markMCQQ.The general solution of the differential equation (x+y)ydx+(y−x)xdy=0 is (A) x+ylog(cy)=0 (B) xy=log(xy)+c (C) x+ylog(cxy)=0 (D) xy=log(cxy)
›Reveal solutionSolution
This is a homogeneous first-order ODE; the substitution y=vx turns it into a separable equation whose integral rearranges to x+ylog(cxy)=0.
Concept and Intuition
Every term of (x+y)ydx+(y−x)xdy=0 is of the same total degree (degree 2 in x,y), which is the signature of a homogeneous differential equation. The standard technique is to set y=vx, turning the equation into one involving only v and x, which separates.
Step-by-Step Solution
- Expand: (xy+y2)dx+(xy−x2)dy=0 — every term has degree 2, confirming homogeneity.
- Put y=vx, so dy=vdx+xdv. Then xy+y2=x2v(1+v) and xy−x2=x2(v−1).
- Substituting: x2v(1+v)dx+x2(v−1)(vdx+xdv)=0. Dividing by x2: v(1+v)dx+(v−1)vdx+x(v−1)dv=0.
- Collect the dx coefficient: v(1+v)+v(v−1)=v[(1+v)+(v−1)]=2v2. So 2v2dx+x(v−1)dv=0.
- Separate variables: xdx=2v2(1−v)dv=(2v21−2v1)dv.
- Integrate: logx=−2v1−21logv+C. Multiply by 2: 2logx+logv=−v1+2C⇒log(x2v)=−v1+K.
- Since x2v=x2⋅xy=xy and v1=yx: log(xy)+yx=K. …
- AP EAPCET 2022Set eng-2022-07-06-AN1 markMCQQ.If cosxy=Alogx+C is the general solution of (xsinxy)dy=(ysinxy−x)dx, then A= (A) 2 (B) 1 (C) -1 (D) -2
›Reveal solutionSolution
The equation is homogeneous; the substitution y=vx reduces it to a separable form whose integration directly gives cos(y/x)=logx+C, so A=1.
Concept and Intuition
The differential equation (xsin(y/x))dy=(ysin(y/x)−x)dx is homogeneous (every term scales the same way under x→λx,y→λy), so the standard substitution y=vx (with v=y/x) converts it into a separable equation in v and x.
Step-by-Step Solution
- Let y=vx, so dy=vdx+xdv.
- Substitute into (xsinv)dy=(vxsinv−x)dx: (xsinv)(vdx+xdv)=(vxsinv−x)dx.
- Expand LHS: vxsinvdx+x2sinvdv=vxsinvdx−xdx.
- The vxsinvdx terms cancel from both sides: x2sinvdv=−xdx.
- Divide by x2 (assuming x=0): sinvdv=−xdx.
- Integrate both sides: −cosv=−log∣x∣+C′⇒cosv=log∣x∣+C. …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.The general solution of the differential equation (x−(x+y)log(x+y))dx+xdy=0 is (A) ylog(x+y)=cx (B) xlog(x+y)=cy (C) log(x+y)=cy (D) log(x+y)=cx
›Reveal solutionSolution
This tests recognizing that regrouping terms via v=x+y converts an awkward mixed equation into a separable one. The general solution is log(x+y)=cx, option (D).
Concept and Intuition
The given equation mixes x, y, and log(x+y) in a way that isn't obviously separable or linear at first glance. The key insight is spotting that dx+dy (which appears once you regroup) is exactly d(x+y) — so substituting v=x+y collapses the two-variable equation into one purely in v and x, which then separates cleanly.
Step-by-Step Solution
- Expand the given equation: xdx−(x+y)log(x+y)dx+xdy=0.
- Regroup: x(dx+dy)=(x+y)log(x+y)dx.
- Let v=x+y, so dv=dx+dy. The equation becomes xdv=vlogvdx.
- Separate variables: vlogvdv=xdx.
- For the left side, let w=logv, dw=vdv, so vlogvdv=wdw, integrating to log∣w∣=log∣logv∣.
- Integrate both sides: log∣logv∣=log∣x∣+C1⇒logv=kx for some constant k (absorbing eC1 and sign). …
- AP EAPCET 2024Set eng-2024-05-20-AN1 markMCQQ.The general solution of the differential equation (xsinxy)dy=(ysinxy−x)dx is (A) cosyx=logex+c (B) cosxy=logex+c (C) cosyx=logey+c (D) cosxy=logey+c
›Reveal solutionSolution
This is a homogeneous DE; the substitution y=vx cleanly separates variables, giving cos(y/x)=logex+c — (B).
Concept and Intuition
An equation is homogeneous when every term has the same total "degree" once you notice it depends only on the ratio y/x. The standard substitution y=vx turns it into a separable equation in v and x.
Step-by-Step Solution
- Given: (xsinxy)dy=(ysinxy−x)dx.
- Let y=vx⇒dy=vdx+xdv, and xy=v.
- Substitute: xsinv(vdx+xdv)=(vxsinv−x)dx.
- Expand LHS: vxsinvdx+x2sinvdv. RHS: vxsinvdx−xdx.
- The vxsinvdx term cancels from both sides, leaving: x2sinvdv=−xdx.
- Divide by x2 (assuming x=0): sinvdv=−xdx.
- Integrate both sides: −cosv=−ln∣x∣+C1⇒cosv=ln∣x∣+C (relabeling the constant).
- Substitute back v=y/x: cosxy=logex+c. …
- AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQQ.The general solution of the differential equation dxdy=2xy+x−4y−22xy−4x+y−2 is (A) 5(y−x)+2log(x−2y−2)=c (B) 2(y−x)−5log(x−2y−2)=c (C) 2(y−x)+5log(x−2y−2)=c (D) 5(y−x)−2log(x−2y−2)=c
›Reveal solutionSolution
This tests solving a separable ODE after factoring both the numerator and denominator by grouping. Answer: (C).
Concept and Intuition
A rational-function differential equation like this often hides a separable form once you notice both numerator and denominator can be grouped into two linear-in-x and linear-in-y factors. Spotting the factoring is the whole trick.
Step-by-Step Solution
- Group the numerator: 2xy−4x+y−2=2x(y−2)+1⋅(y−2)=(y−2)(2x+1).
- Group the denominator: 2xy+x−4y−2=x(2y+1)−2(2y+1)=(2y+1)(x−2).
- So dxdy=(x−2)(2y+1)(y−2)(2x+1), which is separable:
y−22y+1dy=x−22x+1dx.
- Rewrite each side to make integration easy:
y−22y+1=y−22(y−2)+5=2+y−25,x−22x+1=2+x−25.
- Integrate both sides: 2y+5log∣y−2∣=2x+5log∣x−2∣+C. …
- AP EAPCET 2025Set eng-2025-05-27-FN1 markMCQQ.The general solution of the differential equation (xsinxy)dy=(ysinxy−x)dx is (A) cos(xy)=log∣x∣+c (B) cos(xy)=x1+c (C) cos(yx)=log∣y∣+c (D) cosxy=x2+c
›Reveal solutionSolution
The equation is homogeneous in x,y; the substitution y=vx makes it separable and integrates to cos(y/x)=log∣x∣+c.
Concept and Intuition
A differential equation where every term has the same total degree in x,y (here, degree 1, since xsin(y/x) and ysin(y/x) and x are all "degree 1" once y/x is treated as dimensionless) is homogeneous, and the standard move is y=vx: it reduces the two variables x,y to one variable v=y/x plus x, making the equation separable.
Step-by-Step Solution
- Given: (xsin(y/x))dy=(ysin(y/x)−x)dx.
- Let y=vx, so dy=vdx+xdv and y/x=v. Substitute:
xsinv(vdx+xdv)=(vxsinv−x)dx.
- Expand: xvsinvdx+x2sinvdv=xvsinvdx−xdx.
- The xvsinvdx terms on both sides cancel exactly, leaving x2sinvdv=−xdx ⇒ sinvdv=−xdx(x=0). …
- AP EAPCET 2024Set eng-2024-05-22-AN1 markMCQQ.The general solution of the differential equation (xy+y2)dx−(x2−2xy)dy=0 is (A) cxy2=ex/y (B) cxy2ex/y=1 (C) cxyex/y=1 (D) cxy=ex/y
›Reveal solutionSolution
This is a homogeneous first-order ODE; the substitution y=vx solves it, giving cxy2ex/y=1, option (B).
Concept and Intuition
Both (xy+y2) and (x2−2xy) are homogeneous of degree 2 in x,y, so the standard substitution y=vx (turning the equation into one in v and x alone) separates variables.
Step-by-Step Solution
- Write the ODE as (xy+y2)dx=(x2−2xy)dy. Substitute y=vx, dy=vdx+xdv.
- xy+y2=x2(v+v2) and x2−2xy=x2(1−2v). The equation becomes
x2(v+v2)dx=x2(1−2v)(vdx+xdv).
- Divide by x2 and collect dx terms: [(v+v2)−v(1−2v)]dx=(1−2v)xdv. The bracket simplifies to 3v2, so
3v2dx=(1−2v)xdv⇒xdx=3v21−2vdv.
- Integrate: logx=31∫(v21−v2)dv=31(−v1−2logv)+C.
- Multiply by 3: 3logx+2logv+v1=C′, i.e. log(x3v2)=C′−v1, so x3v2=Ke−1/v. …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.The general solution of the differential equation dxdy=2x2+3xy2xy−3y2 is (A) 3logxy=yx+c (B) log∣xy∣=2xy+c (C) 3log∣xy∣=y2x+c (D) logxy=xy+c
›Reveal solutionSolution
The DE is homogeneous of degree 0; substituting y=vx and separating variables leads, after converting back to x,y, to 3log∣xy∣=y2x+c.
Concept and Intuition
A first-order DE dxdy=Q(x,y)P(x,y) is homogeneous when P and Q are both homogeneous of the same degree — here both numerator (2xy−3y2) and denominator (2x2+3xy) are degree 2. The standard technique is the substitution y=vx, which converts the equation into one in v and x alone that separates.
Step-by-Step Solution
- Given: dxdy=2x2+3xy2xy−3y2. Both numerator and denominator are homogeneous of degree 2, so substitute y=vx, dxdy=v+xdxdv.
- Divide numerator and denominator by x2:
2x2+3xy2xy−3y2=2+3v2v−3v2
- So v+xdxdv=2+3v2v−3v2.
- Isolate the derivative term:
xdxdv=2+3v2v−3v2−v=2+3v2v−3v2−v(2+3v)=2+3v2v−3v2−2v−3v2=2+3v−6v2
- Separate variables:
v22+3vdv=x−6dx⟹(v22+v3)dv=−x6dx
- Integrate both sides:
−v2+3log∣v∣=−6log∣x∣+C
- Rearranging: −v2+3log∣v∣+6log∣x∣=C. Since 6log∣x∣=3log(x2), combine logs: …
- AP EAPCET 2022Set eng-2022-07-08-AN1 markMCQQ.The general solution of xdy - ydx = ydy is (A) y=Ae−x/y (B) y=Aex (C) xy=Aex (D) yx+xy=C
›Reveal solutionSolution
The ODE is homogeneous; solving with y=Vx and simplifying the resulting logarithmic relation gives y=Ae−x/y.
Concept and Intuition
"xdy−ydx" style equations are homogeneous in x,y and standardly solved by substituting y=Vx, which turns them into separable equations in V and x.
Step-by-Step Solution
- Rearrange: xdy−ydy=ydx⇒dy(x−y)=ydx⇒dxdy=x−yy.
- Let y=Vx, so dxdy=V+xdxdV. RHS becomes x−VxVx=1−VV.
- So V+xdxdV=1−VV⇒xdxdV=1−VV−V(1−V)=1−VV2.
- Separate: V21−VdV=xdx⇒(V21−V1)dV=xdx.
- Integrate: −V1−logV=logx+C1.
- Substitute back V=y/x: −yx−logxy=logx+C1⇒−yx−logy+logx=logx+C1⇒−yx−logy=C1. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.The general solution of the differential equation dxdy=x2−y22x2−xy−y2 is (A) logx2y2−2x2+2logy+2xy−2x+22log∣x∣=c (B) 2logx2y2−2x2+logy+2xy−2x+22log∣x∣=c (C) 2logx2y2+2x2+logy−2xy+2x+22log∣x∣=c (D) logx22x2−y2+2logy−2xy+2x+log∣x∣=c
›Reveal solutionSolution
A homogeneous ODE solved by y=vx substitution; after partial fractions, the solution is option (B).
Concept and Intuition
The right side x2−y22x2−xy−y2 is a ratio of degree-2 homogeneous polynomials in x,y, so dividing top and bottom by x2 turns everything into a function of v=y/x alone — the standard homogeneous-equation substitution y=vx.
Step-by-Step Solution
- Let y=vx, so dxdy=v+xdxdv. Dividing numerator and denominator of the RHS by x2:
v+xdxdv=1−v22−v−v2.
- Isolate xdv/dx:
xdxdv=1−v22−v−v2−v(1−v2)=1−v2v3−v2−2v+2.
- Factor the cubic: v3−v2−2v+2=(v−1)(v2−2), and 1−v2=−(v−1)(v+1). The (v−1) cancels:
xdxdv=−(v−1)(v+1)(v−1)(v2−2)=v+12−v2.
- Separate variables: 2−v2v+1dv=xdx.
- Partial fractions on 2−v2=(2−v)(2+v): writing (2−v)(2+v)v+1=2−vA+2+vB gives A=42+2, B=42−2.
- Integrating and simplifying logs (combining log∣2−v∣±log∣2+v∣ into log∣2−v2∣ and log2+v2−v terms) and multiplying through, one obtains: 2log∣2−v2∣+log2+v2−v+22log∣x∣=C. …
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.