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Definite Integration question

2007 · Shift 0 · Q46
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Definite Integration question

2007 · Shift 0 · Q46

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
Let I=∫01sin⁡xxdxI = \int\limits_0^1 {{{\sin x} \over {\sqrt x }}dx}I=0∫1​x​sinx​dx and J=∫01cos⁡xxdx.J = \int\limits_0^1 {{{\cos x} \over {\sqrt x }}dx} .J=0∫1​x​cosx​dx. Then which one of the following is true?
  1. A
    1>231 \gt {2 \over 3}1>32​ and J>2J \gt 2J>2
  2. B
    1<231 \lt {2 \over 3}1<32​ and J<2J \lt 2J<2
  3. C
    1<231 \lt {2 \over 3}1<32​ and J>2J \gt 2J>2
  4. D
    1>231 \gt {2 \over 3}1>32​ and J<2J \lt 2J<2
View written solutionFree

Correct answer: B

We need compare I=∫01sin⁡xx dx,J=∫01cos⁡xx dx.I=\int_0^1 \frac{\sin x}{\sqrt x}\,dx,\qquad J=\int_0^1 \frac{\cos x}{\sqrt x}\,dx.I=∫01​x​sinx​dx,J=∫01​x​cosx​dx.

The options are clearly intended to compare III with 23\dfrac2332​ and JJJ with 222.


1. Estimate III

For x∈[0,1]x\in[0,1]x∈[0,1], we use the standard inequality sin⁡x<x(x>0).\sin x < x \quad (x>0).sinx<x(x>0). Therefore, sin⁡xx<xx=x.\frac{\sin x}{\sqrt x} < \frac{x}{\sqrt x}=\sqrt x.x​sinx​<x​x​=x​. Integrating from 000 to 111, I=∫01sin⁡xx dx<∫01x dx.I=\int_0^1 \frac{\sin x}{\sqrt x}\,dx < \int_0^1 \sqrt x\,dx.I=∫01​x​sinx​dx<∫01​x​dx. Now, ∫01x dx=∫01x1/2 dx=[23x3/2]01=23.\int_0^1 \sqrt x\,dx=\int_0^1 x^{1/2}\,dx=\left[\frac{2}{3}x^{3/2}\right]_0^1=\frac23.∫01​x​dx=∫01​x1/2dx=[32​x3/2]01​=32​. Hence, I<23.I<\frac23.I<32​.

So the first comparison is: I<23.I<\frac23.I<32​.


2. Estimate JJJ

For x∈[0,1]x\in[0,1]x∈[0,1], we know cos⁡x<1(x>0).\cos x < 1 \quad (x>0).cosx<1(x>0). Therefore, cos⁡xx<1x.\frac{\cos x}{\sqrt x} < \frac{1}{\sqrt x}.x​cosx​<x​1​. Integrating, J=∫01cos⁡xx dx<∫011x dx.J=\int_0^1 \frac{\cos x}{\sqrt x}\,dx < \int_0^1 \frac{1}{\sqrt x}\,dx.J=∫01​x​cosx​dx<∫01​x​1​dx. Now, ∫011x dx=∫01x−1/2 dx=[2x]01=2.\int_0^1 \frac{1}{\sqrt x}\,dx=\int_0^1 x^{-1/2}\,dx=\left[2\sqrt x\right]_0^1=2.∫01​x​1​dx=∫01​x−1/2dx=[2x​]01​=2. Hence, J<2.J<2.J<2.

So the second comparison is: J<2.J<2.J<2.


3. Match with the options

We found: I<23andJ<2.I<\frac23 \quad \text{and} \quad J<2.I<32​andJ<2. This matches Option B.


4. Comparison with stored correct answer

Stored correct answer: B

Our derived answer: B

So they agree.

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