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Functions question

2024 · Shift 2 · Q27
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Functions question

2024 · Shift 2 · Q27

JEE AdvancedMathematicsFunctionsNumerical+4 / −1
Let the function f:R→Rf: \mathbb{R} \rightarrow \mathbb{R}f:R→R be defined by f(x)=sin⁡xeπx(x2023+2024x+2025)(x2−x+3)+2eπx(x2023+2024x+2025)(x2−x+3).f(x)=\frac{\sin x}{e^{\pi x}} \frac{\left(x^{2023}+2024 x+2025\right)}{\left(x^2-x+3\right)}+\frac{2}{e^{\pi x}} \frac{\left(x^{2023}+2024 x+2025\right)}{\left(x^2-x+3\right)} .f(x)=eπxsinx​(x2−x+3)(x2023+2024x+2025)​+eπx2​(x2−x+3)(x2023+2024x+2025)​. Then the number of solutions of f(x)=0f(x)=0f(x)=0 in R\mathbb{R}R is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 1

  1. Rewrite the function

Given

f(x)=sin⁡xeπx⋅x2023+2024x+2025x2−x+3+2eπx⋅x2023+2024x+2025x2−x+3.f(x)=\frac{\sin x}{e^{\pi x}}\cdot \frac{x^{2023}+2024x+2025}{x^2-x+3} +\frac{2}{e^{\pi x}}\cdot \frac{x^{2023}+2024x+2025}{x^2-x+3}.f(x)=eπxsinx​⋅x2−x+3x2023+2024x+2025​+eπx2​⋅x2−x+3x2023+2024x+2025​.

Factor the common terms:

f(x)=x2023+2024x+2025eπx(x2−x+3)(sin⁡x+2).f(x)=\frac{x^{2023}+2024x+2025}{e^{\pi x}(x^2-x+3)}(\sin x+2).f(x)=eπx(x2−x+3)x2023+2024x+2025​(sinx+2).

So,

f(x)=0  ⟺  x2023+2024x+2025eπx(x2−x+3)(sin⁡x+2)=0.f(x)=0 \iff \frac{x^{2023}+2024x+2025}{e^{\pi x}(x^2-x+3)}(\sin x+2)=0.f(x)=0⟺eπx(x2−x+3)x2023+2024x+2025​(sinx+2)=0.
  1. Check which factors can be zero
  • Since eπx>0e^{\pi x}>0eπx>0 for all x∈Rx\in\mathbb Rx∈R, it is never zero.
  • Consider x2−x+3=(x−12)2+114>0x^2-x+3=\left(x-\frac12\right)^2+\frac{11}{4}>0x2−x+3=(x−21​)2+411​>0 for all real xxx, so this is never zero.
  • Also, −1≤sin⁡x≤1  ⟹  1≤sin⁡x+2≤3,-1\le \sin x \le 1 \implies 1\le \sin x+2 \le 3,−1≤sinx≤1⟹1≤sinx+2≤3, hence sin⁡x+2>0\sin x+2>0sinx+2>0 for all real xxx.

Therefore, the only way f(x)=0f(x)=0f(x)=0 is when

x2023+2024x+2025=0.x^{2023}+2024x+2025=0.x2023+2024x+2025=0.
  1. Find the number of real roots of x2023+2024x+2025=0x^{2023}+2024x+2025=0x2023+2024x+2025=0

Let

g(x)=x2023+2024x+2025.g(x)=x^{2023}+2024x+2025.g(x)=x2023+2024x+2025.

Then

g′(x)=2023x2022+2024.g'(x)=2023x^{2022}+2024.g′(x)=2023x2022+2024.

Since x2022≥0x^{2022}\ge 0x2022≥0 for all real xxx,

g′(x)=2023x2022+2024>0∀x∈R.g'(x)=2023x^{2022}+2024>0 \quad \forall x\in\mathbb R.g′(x)=2023x2022+2024>0∀x∈R.

So g(x)g(x)g(x) is strictly increasing on R\mathbb RR.

Now check end behavior:

lim⁡x→−∞g(x)=−∞,lim⁡x→+∞g(x)=+∞.\lim_{x\to -\infty} g(x)=-\infty, \qquad \lim_{x\to +\infty} g(x)=+\infty.x→−∞lim​g(x)=−∞,x→+∞lim​g(x)=+∞.

A strictly increasing continuous function going from −∞-\infty−∞ to +∞+\infty+∞ crosses the x-axis exactly once.

Hence, the equation

x2023+2024x+2025=0x^{2023}+2024x+2025=0x2023+2024x+2025=0

has exactly one real solution.

  1. Conclusion

Therefore, the number of real solutions of f(x)=0f(x)=0f(x)=0 is 1.\boxed{1}.1​.

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