A curve is defined by the equation y=ln⁡(exx2+10)y = \ln\left(\frac{\text{e}^x}{x^2 + 10}\right). Show that the curve has no turning points for all real values of xx.

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Worked solution

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1. Simplify the equation of the curve using the laws of logarithms.

y=ln⁡(exx2+10)y = \ln\left(\frac{\text{e}^x}{x^2 + 10}\right)
y=ln⁡(ex)−ln⁡(x2+10)y = \ln(\text{e}^x) - \ln(x^2 + 10)
y=x−ln⁡(x2+10)y = x - \ln(x^2 + 10)

2. Differentiate $y$ with respect to $x$.

dydx=1−1x2+10⋅(2x)\frac{dy}{dx} = 1 - \frac{1}{x^2 + 10} \cdot (2x)
dydx=1−2xx2+10\frac{dy}{dx} = 1 - \frac{2x}{x^2 + 10}

3. Equate the derivative to zero to find turning points and form a quadratic equation.

For turning points, dydx=0\frac{dy}{dx} = 0.
1−2xx2+10=01 - \frac{2x}{x^2 + 10} = 0
x2+10−2xx2+10=0\frac{x^2 + 10 - 2x}{x^2 + 10} = 0
x2−2x+10=0x^2 - 2x + 10 = 0

4. Analyse the roots of the quadratic equation using the discriminant.

Discriminant of x2−2x+10=0x^2 - 2x + 10 = 0:
b2−4ac=(−2)2−4(1)(10)b^2 - 4ac = (-2)^2 - 4(1)(10)
=4−40=−36= 4 - 40 = -36
Since −36<0-36 < 0, the equation x2−2x+10=0x^2 - 2x + 10 = 0 has no real roots.
Therefore, dydx=0\frac{dy}{dx} = 0 has no real solutions, meaning the curve has no turning points for all real values of xx.

Answer

Since the discriminant is negative (−36<0-36 < 0), dydx=0\frac{dy}{dx} = 0 has no real roots, hence the curve has no turning points.

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