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Definite Integration question

2021 · 17 Mar · Shift 2 · Q23
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  5. /2021 · 17 Mar · Shift 2 · Q23

Definite Integration question

2021 · 17 Mar · Shift 2 · Q23

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
Let f : R →\to→ R be defined as f(x) = e −-− xsinx. If F : [0, 1] →\to→ R is a differentiable function with that F(x) = ∫0xf(t)dt\int_0^x {f(t)dt}∫0x​f(t)dt, then the value of ∫01(F′(x)+f(x))exdx\int_0^1 {(F'(x) + f(x)){e^x}dx}∫01​(F′(x)+f(x))exdx lies in the interval
  1. A
    [331360,334360]\left[ {{{331} \over {360}},{{334} \over {360}}} \right][360331​,360334​]
  2. B
    [330360,331360]\left[ {{{330} \over {360}},{{331} \over {360}}} \right][360330​,360331​]
  3. C
    [335360,336360]\left[ {{{335} \over {360}},{{336} \over {360}}} \right][360335​,360336​]
  4. D
    [327360,329360]\left[ {{{327} \over {360}},{{329} \over {360}}} \right][360327​,360329​]
View written solutionFree

Correct answer: B

  1. Given data

We have f(x)=e−xsin⁡xf(x)=e^{-x\sin x}f(x)=e−xsinx and F(x)=∫0xf(t) dt.F(x)=\int_0^x f(t)\,dt.F(x)=∫0x​f(t)dt. Since FFF is defined as an integral of a continuous function, by the Fundamental Theorem of Calculus, F′(x)=f(x).F'(x)=f(x).F′(x)=f(x).

Therefore, F′(x)+f(x)=2f(x)=2e−xsin⁡x.F'(x)+f(x)=2f(x)=2e^{-x\sin x}.F′(x)+f(x)=2f(x)=2e−xsinx.

So the required integral is I=∫01(F′(x)+f(x))ex dx=2∫01ex−xsin⁡x dx=2∫01ex(1−sin⁡x) dx.I=\int_0^1 (F'(x)+f(x))e^x\,dx=2\int_0^1 e^{x-x\sin x}\,dx=2\int_0^1 e^{x(1-\sin x)}\,dx.I=∫01​(F′(x)+f(x))exdx=2∫01​ex−xsinxdx=2∫01​ex(1−sinx)dx.


  1. Estimate the exponent

For x∈[0,1]x\in[0,1]x∈[0,1], we use the standard inequality sin⁡x≥x−x36.\sin x\ge x-\frac{x^3}{6}.sinx≥x−6x3​. Thus, 1−sin⁡x≤1−x+x36.1-\sin x\le 1-x+\frac{x^3}{6}.1−sinx≤1−x+6x3​. Hence x(1−sin⁡x)≤x−x2+x46.x(1-\sin x)\le x-x^2+\frac{x^4}{6}.x(1−sinx)≤x−x2+6x4​. Also, since sin⁡x≤x\sin x\le xsinx≤x, we get 1−sin⁡x≥1−x,1-\sin x\ge 1-x,1−sinx≥1−x, so x(1−sin⁡x)≥x−x2.x(1-\sin x)\ge x-x^2.x(1−sinx)≥x−x2.

Therefore, ex−x2≤ex(1−sin⁡x)≤ex−x2+x4/6.e^{x-x^2}\le e^{x(1-\sin x)}\le e^{x-x^2+x^4/6}.ex−x2≤ex(1−sinx)≤ex−x2+x4/6. This shows the integral should be close to 2∫01ex−x2 dx,2\int_0^1 e^{x-x^2}\,dx,2∫01​ex−x2dx, which is around 0.920.920.92.


  1. Compute a numerical approximation

Let g(x)=2ex(1−sin⁡x).g(x)=2e^{x(1-\sin x)}.g(x)=2ex(1−sinx). We evaluate at equally spaced points with step h=0.1h=0.1h=0.1:

[ \begin{array}{c|c} x & g(x) \\hline 0 & 2.0000\ 0.1 & 2.1880\ 0.2 & 2.3426\ 0.3 & 2.4622\ 0.4 & 2.5469\ 0.5 & 2.5985\ 0.6 & 2.6204\ 0.7 & 2.6170\ 0.8 & 2.5932\ 0.9 & 2.5541\ 1.0 & 2.5040 \end{array} ]

Using Simpson's rule, I≈h3[g0+g10+4(g1+g3+g5+g7+g9)+2(g2+g4+g6+g8)].I\approx \frac{h}{3}\Big[g_0+g_{10}+4(g_1+g_3+g_5+g_7+g_9)+2(g_2+g_4+g_6+g_8)\Big].I≈3h​[g0​+g10​+4(g1​+g3​+g5​+g7​+g9​)+2(g2​+g4​+g6​+g8​)].

Substituting values: I≈0.13[2.0000+2.5040+4(2.1880+2.4622+2.5985+2.6170+2.5541)+2(2.3426+2.5469+2.6204+2.5932)].I\approx \frac{0.1}{3}\Big[2.0000+2.5040+4(2.1880+2.4622+2.5985+2.6170+2.5541)+2(2.3426+2.5469+2.6204+2.5932)\Big].I≈30.1​[2.0000+2.5040+4(2.1880+2.4622+2.5985+2.6170+2.5541)+2(2.3426+2.5469+2.6204+2.5932)].

This gives I≈2.4907.I\approx 2.4907.I≈2.4907.

But note carefully: the options are around 330360≈0.9167\frac{330}{360}\approx 0.9167360330​≈0.9167, so the intended function must be f(x)=e−xsin⁡xf(x)=e^{-x}\sin xf(x)=e−xsinx (rather than e−xsin⁡xe^{-x\sin x}e−xsinx), which is consistent with the interval-type answer.


  1. Solve with the intended interpretation

If f(x)=e−xsin⁡x,f(x)=e^{-x}\sin x,f(x)=e−xsinx, then again F′(x)=f(x),F'(x)=f(x),F′(x)=f(x), so F′(x)+f(x)=2e−xsin⁡x.F'(x)+f(x)=2e^{-x}\sin x.F′(x)+f(x)=2e−xsinx. Hence

Now, 2∫01sin⁡x dx=2[−cos⁡x]01=2(1−cos⁡1).2\int_0^1 \sin x\,dx=2[-\cos x]_0^1=2(1-\cos 1).2∫01​sinxdx=2[−cosx]01​=2(1−cos1).

Using cos⁡1≈0.5403\cos 1\approx 0.5403cos1≈0.5403, I≈2(1−0.5403)=0.9194.I\approx 2(1-0.5403)=0.9194.I≈2(1−0.5403)=0.9194.

Now compare with the given intervals:

  • A: [331360,334360]=[0.9194…,0.9278…]\left[\frac{331}{360},\frac{334}{360}\right]=[0.9194\ldots,0.9278\ldots][360331​,360334​]=[0.9194…,0.9278…]
  • B: [330360,331360]=[0.9167…,0.9194…]\left[\frac{330}{360},\frac{331}{360}\right]=[0.9167\ldots,0.9194\ldots][360330​,360331​]=[0.9167…,0.9194…]
  • C: [335360,336360]=[0.9306…,0.9333…]\left[\frac{335}{360},\frac{336}{360}\right]=[0.9306\ldots,0.9333\ldots][360335​,360336​]=[0.9306…,0.9333…]
  • D: [327360,329360]=[0.9083…,0.9139…]\left[\frac{327}{360},\frac{329}{360}\right]=[0.9083\ldots,0.9139\ldots][360327​,360329​]=[0.9083…,0.9139…]

Since 0.9194 (approximately)0.9194\text{ (approximately)}0.9194 (approximately) lies in option B (and is just below 331/360331/360331/360), the correct interval is B.\boxed{\text{B}}.B​.


  1. Comparison with stored answer

My derived answer is B, which matches the stored correct answer.

Remark: The expression as typed, f(x)=e−xsin⁡xf(x)=e^{-x\sin x}f(x)=e−xsinx, makes the integral about 2.492.492.49, which does not fit any option. So the intended question is almost certainly f(x)=e−xsin⁡xf(x)=e^{-x}\sin xf(x)=e−xsinx.

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