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Application of Derivatives question

2024 · 4 Apr · Shift 1 · Q47
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  5. /2024 · 4 Apr · Shift 1 · Q47

Application of Derivatives question

2024 · 4 Apr · Shift 1 · Q47

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
Let the sum of the maximum and the minimum values of the function f(x)=2x2−3x+82x2+3x+8f(x)=\frac{2 x^2-3 x+8}{2 x^2+3 x+8}f(x)=2x2+3x+82x2−3x+8​ be mn\frac{m}{n}nm​, where gcd⁡(m,n)=1\operatorname{gcd}(\mathrm{m}, \mathrm{n})=1gcd(m,n)=1. Then m+n\mathrm{m}+\mathrm{n}m+n is equal to :
  1. A
    217
  2. B
    182
  3. C
    201
  4. D
    195
View written solutionFree

Correct answer: C

  1. Given function

f(x)=2x2−3x+82x2+3x+8f(x)=\frac{2x^2-3x+8}{2x^2+3x+8}f(x)=2x2+3x+82x2−3x+8​

We need the maximum and minimum values of f(x)f(x)f(x) over all real xxx, then add them.


  1. First check domain

Denominator:

2x2+3x+82x^2+3x+82x2+3x+8

Its discriminant is

Δ=32−4⋅2⋅8=9−64=−55<0\Delta=3^2-4\cdot 2\cdot 8=9-64=-55<0Δ=32−4⋅2⋅8=9−64=−55<0

Since the leading coefficient is positive, we have

2x2+3x+8>0for all real x2x^2+3x+8>0 \quad \text{for all real } x2x2+3x+8>0for all real x

So f(x)f(x)f(x) is defined for all real xxx.


  1. Let

y=f(x)=2x2−3x+82x2+3x+8y=f(x)=\frac{2x^2-3x+8}{2x^2+3x+8}y=f(x)=2x2+3x+82x2−3x+8​

To find the range of yyy, cross-multiply:

y(2x2+3x+8)=2x2−3x+8y(2x^2+3x+8)=2x^2-3x+8y(2x2+3x+8)=2x2−3x+8

Rearrange:

2yx2+3yx+8y−2x2+3x−8=02yx^2+3yx+8y-2x^2+3x-8=02yx2+3yx+8y−2x2+3x−8=0

2(y−1)x2+3(y+1)x+8(y−1)=02(y-1)x^2+3(y+1)x+8(y-1)=02(y−1)x2+3(y+1)x+8(y−1)=0

This is a quadratic in xxx. For real xxx, its discriminant must be non-negative.


  1. Apply discriminant condition

For

2(y−1)x2+3(y+1)x+8(y−1)=02(y-1)x^2+3(y+1)x+8(y-1)=02(y−1)x2+3(y+1)x+8(y−1)=0

we need

[3(y+1)]2−4⋅2(y−1)⋅8(y−1)≥0[3(y+1)]^2-4\cdot 2(y-1)\cdot 8(y-1)\ge 0[3(y+1)]2−4⋅2(y−1)⋅8(y−1)≥0

9(y+1)2−64(y−1)2≥09(y+1)^2-64(y-1)^2\ge 09(y+1)2−64(y−1)2≥0

Expand:

9(y2+2y+1)−64(y2−2y+1)≥09(y^2+2y+1)-64(y^2-2y+1)\ge 09(y2+2y+1)−64(y2−2y+1)≥0

9y2+18y+9−64y2+128y−64≥09y^2+18y+9-64y^2+128y-64\ge 09y2+18y+9−64y2+128y−64≥0

−55y2+146y−55≥0-55y^2+146y-55\ge 0−55y2+146y−55≥0

Multiply by −1-1−1:

55y2−146y+55≤055y^2-146y+55\le 055y2−146y+55≤0

So yyy lies between the roots of

55y2−146y+55=055y^2-146y+55=055y2−146y+55=0


  1. Find the roots

y=146±1462−4⋅55⋅552⋅55y=\frac{146\pm\sqrt{146^2-4\cdot 55\cdot 55}}{2\cdot 55}y=2⋅55146±1462−4⋅55⋅55​​

=146±21316−12100110=\frac{146\pm\sqrt{21316-12100}}{110}=110146±21316−12100​​

=146±9216110=\frac{146\pm\sqrt{9216}}{110}=110146±9216​​

=146±96110=\frac{146\pm 96}{110}=110146±96​

Thus the two roots are

ymin⁡=146−96110=50110=511y_{\min}=\frac{146-96}{110}=\frac{50}{110}=\frac{5}{11}ymin​=110146−96​=11050​=115​

and

ymax⁡=146+96110=242110=115y_{\max}=\frac{146+96}{110}=\frac{242}{110}=\frac{11}{5}ymax​=110146+96​=110242​=511​

Hence,

min⁡f(x)=511,max⁡f(x)=115\min f(x)=\frac{5}{11}, \qquad \max f(x)=\frac{11}{5}minf(x)=115​,maxf(x)=511​


  1. Sum of maximum and minimum values

115+511=121+2555=14655\frac{11}{5}+\frac{5}{11}=\frac{121+25}{55}=\frac{146}{55}511​+115​=55121+25​=55146​

So,

m=146,n=55m=146,\quad n=55m=146,n=55

and

m+n=146+55=201m+n=146+55=201m+n=146+55=201


  1. Option check

201201201 corresponds to Option C.


  1. Comparison with stored correct answer

Stored correct answer: C

Our derived answer: C

So they agree.

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