A curve has equation y=f(x)y=f(x), where f(x)=x3+ax2+bx+cf(x)=x^{3}+ax^{2}+bx+c. The curve y=f(x)y=f(x) has a maximum point at (1,−32)(1,-32), while the curve y=1f(x)y=\dfrac{1}{f(x)} has a vertical asymptote with equation x=5x=5. Find the equation of the curve y=f(x)y=f(x).

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Worked solution

1

1. Express the condition that the curve has a stationary point at $x=1$ as an equation in terms of $a$ and $b$.

f′(x)=3x2+2ax+bf'(x) = 3x^2 + 2ax + b
At the stationary point (1,−32)(1, -32), f′(1)=0f'(1) = 0:
3(1)2+2a(1)+b=03(1)^2 + 2a(1) + b = 0
2a+b=−3— (1)2a + b = -3 \quad \text{--- (1)}

2. Express the condition that the curve passes through the maximum point $(1, -32)$ as an equation in terms of $a, b$, and $c$.

Since the curve passes through (1,−32)(1, -32), we have f(1)=−32f(1) = -32:
(1)3+a(1)2+b(1)+c=−32(1)^3 + a(1)^2 + b(1) + c = -32
1+a+b+c=−321 + a + b + c = -32
a+b+c=−33— (2)a + b + c = -33 \quad \text{--- (2)}

3. Relate the vertical asymptote of $y = \frac{1}{f(x)}$ to the roots of $f(x)$ to form a third equation.

For the curve y=1f(x)y = \frac{1}{f(x)} to have a vertical asymptote at x=5x = 5, the denominator must be zero at this point. Hence, f(5)=0f(5) = 0.
(5)3+a(5)2+b(5)+c=0(5)^3 + a(5)^2 + b(5) + c = 0
125+25a+5b+c=0125 + 25a + 5b + c = 0
25a+5b+c=−125— (3)25a + 5b + c = -125 \quad \text{--- (3)}

4. Solve the system of linear equations simultaneously to find $a, b$, and $c$, and state the equation of the curve.

From (1), we can express bb in terms of aa:
b=−3−2ab = -3 - 2a
Substitute bb into (2):
a+(−3−2a)+c=−33a + (-3 - 2a) + c = -33
−a−3+c=−33  ⟹  c=a−30-a - 3 + c = -33 \implies c = a - 30
Substitute both bb and cc into (3):
25a+5(−3−2a)+(a−30)=−12525a + 5(-3 - 2a) + (a - 30) = -125
25a−15−10a+a−30=−12525a - 15 - 10a + a - 30 = -125
16a−45=−12516a - 45 = -125
16a=−80  ⟹  a=−516a = -80 \implies a = -5
Substitute a=−5a = -5 back into the expressions for bb and cc:
b=−3−2(−5)=−3+10=7b = -3 - 2(-5) = -3 + 10 = 7
c=−5−30=−35c = -5 - 30 = -35
Thus, the equation of the curve is y=x3−5x2+7x−35y = x^3 - 5x^2 + 7x - 35.

Answer

y=x3−5x2+7x−35y = x^3 - 5x^2 + 7x - 35

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