A curve CC has equation f(x)=ax2+bx+c\text{f}(x) = \frac{a}{x^2} + bx + c, where aa, bb and cc are constants. It is given that CC passes through the points (−1,4)(-1, 4) and (2,−11)(2, -11) and the curve of y=1f(x)y = \frac{1}{\text{f}(x)} has a vertical asymptote with equation x=1x = 1. Find the equation of CC.

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Worked solution

3

1. Interpret the vertical asymptote of the reciprocal graph to form the first equation.

Since the curve y=1f(x)y = \frac{1}{\text{f}(x)} has a vertical asymptote at x=1x = 1, its denominator must equal 00 when x=1x = 1.

f(1)=0\text{f}(1) = 0
a12+b(1)+c=0\frac{a}{1^2} + b(1) + c = 0
a+b+c=0— (1)a + b + c = 0 \quad \text{--- (1)}

2. Use the given points on the curve $C$ to form two more equations.

Given that CC passes through (−1,4)(-1, 4):
f(−1)=4\text{f}(-1) = 4
a(−1)2+b(−1)+c=4\frac{a}{(-1)^2} + b(-1) + c = 4
a−b+c=4— (2)a - b + c = 4 \quad \text{--- (2)}

Given that CC passes through (2,−11)(2, -11):
f(2)=−11\text{f}(2) = -11
a22+b(2)+c=−11\frac{a}{2^2} + b(2) + c = -11
a4+2b+c=−11\frac{a}{4} + 2b + c = -11
a+8b+4c=−44— (3)a + 8b + 4c = -44 \quad \text{--- (3)}

3. Solve the system of linear equations to find $a$, $b$, and $c$.

Subtracting (1) from (2) eliminates aa and cc:
(a−b+c)−(a+b+c)=4−0(a - b + c) - (a + b + c) = 4 - 0
−2b=4  ⟹  b=−2-2b = 4 \implies b = -2

Substitute b=−2b = -2 into (1):
a−2+c=0  ⟹  c=2−aa - 2 + c = 0 \implies c = 2 - a

Substitute b=−2b = -2 and c=2−ac = 2 - a into (3):
a+8(−2)+4(2−a)=−44a + 8(-2) + 4(2 - a) = -44
a−16+8−4a=−44a - 16 + 8 - 4a = -44
−3a−8=−44-3a - 8 = -44
−3a=−36  ⟹  a=12-3a = -36 \implies a = 12

Substitute a=12a = 12 into c=2−ac = 2 - a:
c=2−12=−10c = 2 - 12 = -10

4. State the final equation of the curve $C$.

Substituting a=12a = 12, b=−2b = -2, and c=−10c = -10 back into the original form for CC:
y=12x2−2x−10y = \frac{12}{x^2} - 2x - 10

Answer

y=12x2−2x−10y = \frac{12}{x^2} - 2x - 10

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