Question

If \( 5 \sinh x - \cosh x = 5 \), then one of the values of \( \tanh x \) is:

• A. \( \frac{2}{5} \)
• B. \( \frac{3}{5} \)
• C. \( \frac{-3}{5} \)
• D. \( \frac{-1}{5} \)

Hint:

When solving for \( \tanh x \), use the relations for hyperbolic sine and cosine, and solve the quadratic equation carefully. Verify the solutions using the definition of \( \tanh x \).

Answer 1

The Correct Option is C

We are given the equation: \[ 5 \sinh x - \cosh x = 5. \] Recall the definitions of the hyperbolic sine and cosine functions: \[ \sinh x = \frac{e^x - e^{-x}}{2}, \quad \cosh x = \frac{e^x + e^{-x}}{2}. \] Substitute these definitions into the given equation: \[ 5 \left( \frac{e^x - e^{-x}}{2} \right) - \left( \frac{e^x + e^{-x}}{2} \right) = 5. \] Simplify the terms: \[ \frac{5(e^x - e^{-x})}{2} - \frac{(e^x + e^{-x})}{2} = 5. \] Factor out \( \frac{1}{2} \): \[ \frac{1}{2} \left( 5(e^x - e^{-x}) - (e^x + e^{-x}) \right) = 5. \] Simplify the terms inside the parentheses: \[ \frac{1}{2} \left( 5e^x - 5e^{-x} - e^x - e^{-x} \right) = 5. \] \[ \frac{1}{2} \left( 4e^x - 6e^{-x} \right) = 5. \] Multiply both sides by 2: \[ 4e^x - 6e^{-x} = 10. \] Now, divide through by 2: \[ 2e^x - 3e^{-x} = 5. \] Step 2: Solve for \( \tanh x \).
Let \( y = e^x \), so that \( e^{-x} = \frac{1}{y} \). Substitute these into the equation: \[ 2y - 3\left( \frac{1}{y} \right) = 5. \] Multiply through by \( y \): \[ 2y^2 - 3 = 5y. \] Rearrange this into a standard quadratic form: \[ 2y^2 - 5y - 3 = 0. \] Step 3: Solve the quadratic equation.
Solve the quadratic equation using the quadratic formula: \[ y = \frac{-(-5) \pm \sqrt{(-5)^2 - 4(2)(-3)}}{2(2)} = \frac{5 \pm \sqrt{25 + 24}}{4} = \frac{5 \pm \sqrt{49}}{4} = \frac{5 \pm 7}{4}. \] Thus, the two possible solutions for \( y \) are: \[ y = \frac{5 + 7}{4} = 3 \quad \text{or} \quad y = \frac{5 - 7}{4} = -\frac{1}{2}. \] Step 4: Find \( \tanh x \).
Recall that: \[ \tanh x = \frac{\sinh x}{\cosh x} = \frac{e^x - e^{-x}}{e^x + e^{-x}}. \] Using the values of \( y = e^x \) and \( e^{-x} = \frac{1}{y} \), we have: \[ \tanh x = \frac{y - \frac{1}{y}}{y + \frac{1}{y}}. \] For \( y = 3 \): \[ \tanh x = \frac{3 - \frac{1}{3}}{3 + \frac{1}{3}} = \frac{\frac{8}{3}}{\frac{10}{3}} = \frac{8}{10} = \frac{4}{5}. \] For \( y = -\frac{1}{2} \): \[ \tanh x = \frac{-\frac{1}{2} - 2}{-\frac{1}{2} + 2} = \frac{-\frac{5}{2}}{\frac{3}{2}} = -\frac{5}{3}. \] Thus, the correct value of \( \tanh x \) is \( \boxed{-\frac{3}{5}} \).