3 Solve in \( \mathbb{R} \) each of the following equations : 1) \( e^{3 x+1}=e^{2 x-2} \) 2) \( e^{x^{2}} \cdot e^{x}=1 \) 3) \( \frac{2 e^{x}+1}{e^{x}+1}>0 \) 4) \( e^{2 x}-e^{x}-6 \geq 0 \)
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The Deep Dive
Let's solve each equation step by step: 1) **Equation**: \( e^{3x+1} = e^{2x-2} \) To solve this, we can take the natural logarithm of both sides: \[ 3x + 1 = 2x - 2 \] Now rearranging the equation: \[ 3x - 2x = -2 - 1 \] \[ x = -3 \] **Solution**: \( x = -3 \) --- 2) **Equation**: \( e^{x^2} \cdot e^{x} = 1 \) Using the property of exponents, we can combine them: \[ e^{x^2 + x} = 1 \] The only time the exponential function equals 1 is when its exponent is 0: \[ x^2 + x = 0 \] Factoring the equation gives: \[ x(x + 1) = 0 \] This results in: \[ x = 0 \quad \text{or} \quad x = -1 \] **Solutions**: \( x = 0, -1 \) --- 3) **Inequality**: \( \frac{2 e^{x} + 1}{e^{x} + 1} > 0 \) The inequality is positive when the numerator and denominator are both positive or both negative. The denominator \( e^{x} + 1 \) is always positive for all \( x \) since \( e^{x} > 0 \). Therefore, we only need to check when the numerator is positive: \[ 2e^{x} + 1 > 0 \] This inequality is also true for all \( x \) because \( 2e^{x} > -1 \) for all real \( x \). **Conclusion**: The inequality holds for all \( x \in \mathbb{R} \). --- 4) **Inequality**: \( e^{2x} - e^{x} - 6 \geq 0 \) Let \( y = e^{x} \). The inequality becomes: \[ y^2 - y - 6 \geq 0 \] Factoring gives: \[ (y - 3)(y + 2) \geq 0 \] Finding the roots: \[ y = 3 \quad \text{and} \quad y = -2 \] Since \( y = e^{x} > 0 \), we only consider \( y = 3 \) for the inequality. Using a sign chart, the intervals to test are \( (-\infty, -2) \), \( (-2, 3) \), and \( (3, \infty) \). We find that the inequality is satisfied in the intervals: \[ y \leq -2 \quad \text{or} \quad y \geq 3 \] Since \( y = e^{x} > 0 \), we conclude that: \[ e^{x} \geq 3 \] Taking the natural logarithm: \[ x \geq \ln(3) \] **Solution**: \( x \geq \ln(3) \)