SS E - The number of real solutions of the equation e** + 4e3* - 58e2* + 4X +1 = 0 is P 2a +

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Published June 26, 2025
Mathematics
Algebra
Exponential Equations
Polynomials (Palindromic)

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Detailed Explanation

1. Recognising Exponential–Polynomial Mix

The equation e4x+4e3x58e2x+4ex+1=0e^{4x} + 4e^{3x} - 58e^{2x} + 4e^{x} + 1 = 0 contains powers of exe^{x}. Because exe^{x} is always positive, we can safely set t=ex(t>0).t = e^{x} \quad (t>0). This converts the exponential equation to a pure polynomial:

t4+4t358t2+4t+1=0.t^{4} + 4t^{3} - 58t^{2} + 4t + 1 = 0.

2. Spotting Palindromic Structure

A polynomial is called palindromic when its coefficients read the same forward and backward. Dividing by t2t^{2} (which is allowed since t>0t>0) gives a symmetric expression in tt and 1t\dfrac1t:

t2+4t58+41t+1t2=0.t^{2} + 4t - 58 + 4\frac1t + \frac1{t^{2}} = 0.

Group the terms via the identity t2+1t2=(t+1t)22.t^{2} + \frac1{t^{2}} = \left(t + \frac1t\right)^2 - 2. Define u=t+1t.u = t + \frac1t.

Then the equation reduces to a quadratic in uu:

u2+4u60=0.u^{2} + 4u - 60 = 0.

3. Solving the Quadratic in uu

Solve using the quadratic formula:

= \frac{-4 \pm \sqrt{256}}{2} = \frac{-4 \pm 16}{2}.$$ So $$u = 6 \quad\text{or}\quad u = -10.$$ ### 4. Feasibility Check Because $t>0$ we always have $t + \dfrac1t \ge 2$ (AM ≥ GM). Therefore * $u = 6$ is **allowed**. * $u = -10$ is **impossible**. ### 5. Returning to $t$ For $u = 6$: $$t + \frac1t = 6 \;\Longrightarrow\; t^{2} - 6t + 1 = 0.$$ Solve: $$t = 3 \pm 2\sqrt2.$$ Both roots are positive (\(3+2\sqrt2\approx5.828\); \(3-2\sqrt2\approx0.172\)). ### 6. Back to $x$ Since $t=e^{x}$, $$x_1 = \ln\!(3 + 2\sqrt2), \qquad x_2 = \ln\!(3 - 2\sqrt2).$$ Both are real. No other $t$ values exist, so **exactly two real $x$** satisfy the original equation.

Simple Explanation (ELI5)

What is the question?

We are asked how many real‐number answers (roots) satisfy the funny looking equation made with the exponential function exe^x:

e4x+4e3x58e2x+4ex+1=0e^{4x} + 4e^{3x} - 58e^{2x} + 4e^{x} + 1 = 0

Idea in kid language

  1. Change the scary thing. Every time we see exe^{x} we put a new simpler letter tt (because exe^{x} is always positive).
  2. The giant equation becomes a simpler fourth‐degree (quartic) rule in tt.
  3. Notice the numbers in that new rule read the same forwards and backwards (palindromic). That special symmetry lets us shrink it to a second‐degree (quadratic) rule in t+1tt + \dfrac1t.
  4. Solve the small quadratic, check which answers are possible (remember t>0t>0), then turn them back to xx using x=lntx = \ln t.
  5. Count how many xx values come out.

When you do this carefully you get exactly two real answers.

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Step-by-Step Solution

Step-by-Step Solution

  1. Substitution
    t=ex(t>0)t = e^{x} \quad (t>0) gives t4+4t358t2+4t+1=0.t^{4} + 4t^{3} - 58t^{2} + 4t + 1 = 0.

  2. Divide by t2t^{2} (allowed since t>0t>0): t2+4t58+4t1+t2=0.t^{2} + 4t - 58 + 4t^{-1} + t^{-2} = 0.

  3. Use symmetry with u=t+t1u = t + t^{-1}:

    t2+t2=(t+t1)22=u22.t^{2} + t^{-2} = (t + t^{-1})^{2} - 2 = u^{2} - 2.

    Plug in: (u22)+4u58=0    u2+4u60=0.(u^{2} - 2) + 4u - 58 = 0 \;\Longrightarrow\; u^{2} + 4u - 60 = 0.

  4. Solve the quadratic: u=4±16+2402=4±162.u = \frac{-4 \pm \sqrt{16 + 240}}{2} = \frac{-4 \pm 16}{2}. u1=6,u2=10.u_1 = 6, \qquad u_2 = -10.

  5. Feasibility (t>0u2t>0 \Rightarrow u \ge 2): Reject u=10u=-10. Keep u=6u=6.

  6. Recover tt from u=6u=6: t+1t=6    t26t+1=0.t + \frac1t = 6 \;\Longrightarrow\; t^{2} - 6t + 1 = 0. t=3±22.t = 3 \pm 2\sqrt2. Both tt values are positive.

  7. Return to xx using x=lntx = \ln t:

    x1=ln ⁣(3+22),x2=ln ⁣(322).x_1 = \ln\!(3 + 2\sqrt2), \qquad x_2 = \ln\!(3 - 2\sqrt2).

  8. Count roots: Two real xx values.

Answer: Number of real solutions =2.= \boxed{2}.

Examples

Example 1

Optics: Refractive index decays as e^{kx}. Sometimes equations for darkness points reduce to polynomial in e^{kx}.

Example 2

Electrical engineering: The transient response of an RLC circuit can lead to e^{st} terms. Symmetric characteristic equations allow similar tricks.

Example 3

Population dynamics: Logistic growth with harvesting can create equations where substituting e^{kx} simplifies analysis.

Visual Representation

References

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