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If f(x)=alog⁡∣x∣+bx+xf(x) = a\log|x| + \dfrac{b}{x} + x has its extreme values at x=−1x = -1 and x=3x = 3, then (a,b)(a, b) is equal to:

Solution

✅ Correct Option: 4

Differentiate f(x)f(x):

f′(x)=ax−bx2+1f'(x) = \frac{a}{x} - \frac{b}{x^2} + 1

At an extreme value, f′(x)=0f'(x) = 0. Multiply through by x2x^2:

x2+ax−b=0x^2 + ax - b = 0

The roots of this quadratic are x=−1x = -1 and x=3x = 3.

Sum of roots:

−a=−1+3=2  ⟹  a=−2-a = -1 + 3 = 2 \implies a = -2

Product of roots:

−b=(−1)(3)=−3  ⟹  b=3-b = (-1)(3) = -3 \implies b = 3

Verification:

f′(x)=−2x−3x2+1=x2−2x−3x2=(x−3)(x+1)x2f'(x) = -\frac{2}{x} - \frac{3}{x^2} + 1 = \frac{x^2 - 2x - 3}{x^2} = \frac{(x-3)(x+1)}{x^2}

This equals 00 at x=−1x = -1 and x=3x = 3. ✓

∴(a,b)=(−2,3)\therefore (a, b) = (-2, 3)

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