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← JEE Main 2022 · 24 Jun · Shift 1
MathematicsSingle correct+4 / -1Question 18 of 90

JEE Main 2022 · 24 Jun · Shift 1, Mathematics question 18

The domain of the function f(x)=cos⁡−1(x2−5x+6x2−9)log⁡e(x2−3x+2)f(x)=\dfrac{\cos^{-1}\left(\dfrac{x^2-5x+6}{x^2-9}\right)}{\log_e\left(x^2-3x+2\right)} is :

Choose one option
Answer and worked solution

Official answer: Dropped: full marks to everyone

This question was dropped in NTA's final key. The working shows why no option is right.

Logarithm. x2−3x+2>0x^2-3x+2>0 needs x<1x<1 or x>2x>2, and the denominator is non-zero only if x2−3x+2≠1x^2-3x+2\ne1, i.e. x≠3±52x\ne\dfrac{3\pm\sqrt5}2.

Inverse cosine. For x≠3x\ne3 the argument is x−2x+3\dfrac{x-2}{x+3} (and x=3x=3 is excluded since the denominator x2−9x^2-9 vanishes). −1≤x−2x+3≤1-1\le\dfrac{x-2}{x+3}\le1 gives x≥−12x\ge-\frac12.

Domain. [−12,1)∪(2,∞)−{3,3±52}\left[-\frac12,1\right)\cup(2,\infty)-\left\{3,\dfrac{3\pm\sqrt5}2\right\}. Option (D) omits the exclusion of x=3x=3, and the others are wider still, so none matches.