Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Definite Integration question

2009 · Shift 2 · Q20
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Advanced
  3. /Mathematics
  4. /Definite Integration
  5. /2009 · Shift 2 · Q20

Definite Integration question

2009 · Shift 2 · Q20

JEE AdvancedMathematicsDefinite IntegrationNumerical+3 / −1
Let f:R→Rf:R \to Rf:R→R be a continuous function which satisfies f(x)=∫0xf(t)dtf(x) = \int\limits_0^x {f(t)dt}f(x)=0∫x​f(t)dt. Then, the value of f(ln⁡5)f(\ln 5)f(ln5) is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 0

The user wants me to solve a problem involving a functional equation with an integral.

1. Understand the given functional equation: We are given a continuous function f:R→Rf: R \to Rf:R→R that satisfies the equation: f(x)=∫0xf(t)dtf(x) = \int\limits_0^x {f(t)dt}f(x)=0∫x​f(t)dt
This equation must hold for all x∈Rx \in Rx∈R.

2. Find an initial condition: Let's evaluate the equation at x=0x=0x=0. f(0)=∫00f(t)dtf(0) = \int\limits_0^0 {f(t)dt}f(0)=0∫0​f(t)dt The definite integral from a point to itself is always zero. Therefore, f(0)=0f(0) = 0f(0)=0 This gives us an initial condition for the function f(x)f(x)f(x).

3. Form a differential equation: Since f(x)f(x)f(x) is given to be continuous, we can differentiate both sides of the given functional equation with respect to xxx. We will use the Leibniz rule (or the first Fundamental Theorem of Calculus) for differentiating the integral part.

According to the Fundamental Theorem of Calculus, if F(x)=∫axg(t)dtF(x) = \int_a^x g(t)dtF(x)=∫ax​g(t)dt, then F′(x)=g(x)F'(x) = g(x)F′(x)=g(x) for a continuous function ggg.

Applying this to our equation: ddxf(x)=ddx(∫0xf(t)dt)\frac{d}{dx} f(x) = \frac{d}{dx} \left( \int\limits_0^x {f(t)dt} \right)dxd​f(x)=dxd​(0∫x​f(t)dt)
f′(x)=f(x)f'(x) = f(x)f′(x)=f(x)
This is a first-order linear ordinary differential equation.

4. Solve the differential equation: We need to solve the differential equation f′(x)=f(x)f'(x) = f(x)f′(x)=f(x), or dydx=y\frac{dy}{dx} = ydxdy​=y where y=f(x)y = f(x)y=f(x). We can solve this by separating variables: dyy=dx\frac{dy}{y} = dxydy​=dx
Integrating both sides: ∫dyy=∫dx\int \frac{dy}{y} = \int dx∫ydy​=∫dx
ln⁡∣y∣=x+C1\ln|y| = x + C_1ln∣y∣=x+C1​
where C1C_1C1​ is the constant of integration.

To solve for yyy, we exponentiate both sides: ∣y∣=ex+C1=ex⋅eC1|y| = e^{x+C_1} = e^x \cdot e^{C_1}∣y∣=ex+C1​=ex⋅eC1​
y=±eC1exy = \pm e^{C_1} e^xy=±eC1​ex
Let C=±eC1C = \pm e^{C_1}C=±eC1​. Then the general solution for f(x)f(x)f(x) is: f(x)=Cexf(x) = Ce^xf(x)=Cex
where CCC is an arbitrary constant.

5. Determine the constant C: We use the initial condition we found in step 2, which is f(0)=0f(0) = 0f(0)=0. Let's substitute x=0x=0x=0 into our general solution: f(0)=Ce0f(0) = Ce^0f(0)=Ce0
0=C⋅10 = C \cdot 10=C⋅1
C=0C = 0C=0

6. Determine the function f(x): Substituting the value of C=0C=0C=0 back into the general solution f(x)=Cexf(x) = Ce^xf(x)=Cex, we get: f(x)=0⋅ex=0f(x) = 0 \cdot e^x = 0f(x)=0⋅ex=0
So, the only continuous function that satisfies the given equation is the zero function, f(x)=0f(x) = 0f(x)=0 for all x∈Rx \in Rx∈R.

7. Calculate the required value: The question asks for the value of f(ln⁡5)f(\ln 5)f(ln5). Since f(x)=0f(x) = 0f(x)=0 for all xxx, we have: f(ln⁡5)=0f(\ln 5) = 0f(ln5)=0

Final Answer: The value of f(ln⁡5)f(\ln 5)f(ln5) is 0.

PreviousNext

More from Definite Integration

  • If In​=−π∫π​(1+πx)sinxsinnx​dx,n=0,1,2, .... then2009 · Multiple correct
  • Consider the functions defined implicitly by the equation y3−3y+x=0 on various intervals in the real line. If x∈(−∞,−2)∪(2,∞), the equation implicitly defines a unique real valued differentiable function y=f(x). If x∈(−2,2)…2008 · MCQ
  • Consider the function f:(−∞,∞)→(−∞,∞) defined by f(x)=x2+ax+1x2−ax+1​,0<a<2.Let g(x)=0∫ex​1+t2f′(t)​dt.…2008 · MCQ
  • x→4π​lim​x2−16π2​2∫sec2x​f(t)dt​ equal2007 · MCQ
  • Match the integrals in Column I with the values in Column II. Includes table2007 · MCQ
  • If α=21​∫2​2x2−3x+2tan−1x​dx then the value of 7​tan(π2α7​​) is ​. (Here, the inverse trigonometric function tan−1x…2025 · Numerical
  • Let f:[0,2π​]→[0,1] be the function defined by f(x)=sin2x and let g:[0,2π​]→[0,∞) be the function defined by g(x)=2πx​−x2​.The value of 20∫2π​​f(x)g(x)dx−0∫2π​​g(x)dx…2024 · Numerical
  • Let f:[0,2π​]→[0,1] be the function defined by f(x)=sin2x and let g:[0,2π​]→[0,∞) be the function defined by g(x)=2πx​−x2​.The value of π316​0∫2π​​f(x)g(x)dx…2024 · Numerical