The equation of a curve is y=ex2−5xy = e^{x^2 - 5x}.

  1. (i)

    Find the expressions for dydx\frac{dy}{dx} and d2ydx2\frac{d^2y}{dx^2}.

  2. (ii)

    Find the exact values of the coordinates of the stationary point.

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Worked solution

6(i)

1. Differentiate $y$ with respect to $x$ using the chain rule

dydx=ex2−5x⋅ddx(x2−5x)\frac{dy}{dx} = e^{x^2 - 5x} \cdot \frac{d}{dx}(x^2 - 5x)
dydx=(2x−5)ex2−5x\frac{dy}{dx} = (2x - 5)e^{x^2 - 5x}

2. Differentiate $\frac{dy}{dx}$ with respect to $x$ using the product rule

Let u=2x−5u = 2x - 5 and v=ex2−5xv = e^{x^2 - 5x}.
dudx=2\frac{du}{dx} = 2
dvdx=(2x−5)ex2−5x\frac{dv}{dx} = (2x - 5)e^{x^2 - 5x}
d2ydx2=udvdx+vdudx\frac{d^2y}{dx^2} = u \frac{dv}{dx} + v \frac{du}{dx}
d2ydx2=(2x−5)(2x−5)ex2−5x+ex2−5x(2)\frac{d^2y}{dx^2} = (2x - 5)(2x - 5)e^{x^2 - 5x} + e^{x^2 - 5x}(2)

3. Simplify the expression for the second derivative

d2ydx2=[(2x−5)2+2]ex2−5x\frac{d^2y}{dx^2} = [(2x - 5)^2 + 2]e^{x^2 - 5x}
d2ydx2=(4x2−20x+25+2)ex2−5x\frac{d^2y}{dx^2} = (4x^2 - 20x + 25 + 2)e^{x^2 - 5x}
d2ydx2=(4x2−20x+27)ex2−5x\frac{d^2y}{dx^2} = (4x^2 - 20x + 27)e^{x^2 - 5x}

Answer

dydx=(2x−5)ex2−5x\frac{dy}{dx} = (2x - 5)e^{x^2 - 5x}
d2ydx2=(4x2−20x+27)ex2−5x\frac{d^2y}{dx^2} = (4x^2 - 20x + 27)e^{x^2 - 5x}

6(ii)

1. Set the first derivative to zero to find the $x$-coordinate of the stationary point

From part (i), dydx=(2x−5)ex2−5x\frac{dy}{dx} = (2x - 5)e^{x^2 - 5x}.
For a stationary point, dydx=0\frac{dy}{dx} = 0.
(2x−5)ex2−5x=0(2x - 5)e^{x^2 - 5x} = 0

2. Solve for $x$

Since ex2−5x>0e^{x^2 - 5x} > 0 for all real xx, we have:
2x−5=02x - 5 = 0
x=52x = \frac{5}{2}

3. Find the corresponding $y$-coordinate

Substitute x=52x = \frac{5}{2} into the equation of the curve:
y=e(52)2−5(52)y = e^{(\frac{5}{2})^2 - 5(\frac{5}{2})}
y=e254−252y = e^{\frac{25}{4} - \frac{25}{2}}
y=e−254y = e^{-\frac{25}{4}}

4. State the exact coordinates of the stationary point

The coordinates of the stationary point are (52,e−254)(\frac{5}{2}, e^{-\frac{25}{4}}).

Answer

(52,e−254)(\frac{5}{2}, e^{-\frac{25}{4}})

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