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Question 116 of 143

Q.If f(x)=x2−3xf(x)=x^2-3x, then the points at which f(x)=f′(x)f(x)=f'(x) are:

(a) both irrational
(b) one rational and another irrational
(c) both positive integers
(d) both negative integers
Puducherry TnboardTamil Nadu HSC First Year (DGE) Board 2019MCQ· 1mImportance★★★★★
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Solving x2−3x=2x−3x^2-3x = 2x-3 leads to x2−5x+3=0x^2-5x+3=0, whose discriminant 1313 is not a perfect square, so both roots 5±132\dfrac{5\pm\sqrt{13}}{2} are irrational.

f(x)=x2−3xf(x)=x^2-3x, so f′(x)=2x−3f'(x)=2x-3.

Set f(x)=f′(x)f(x)=f'(x): x2−3x=2x−3x^2-3x = 2x-3.

Rearrange: x2−3x−2x+3=0⇒x2−5x+3=0x^2-3x-2x+3 = 0 \Rightarrow x^2-5x+3=0.

Discriminant =(−5)2−4(1)(3)=25−12=13= (-5)^2-4(1)(3) = 25-12 = 13, which is not a perfect square. …

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