The graph of y=ax2+bx+cx+dy = \frac{ax^2 + bx + c}{x+d} has a vertical asymptote with equation x=−1x = -1 and an oblique asymptote with equation y=2x+2y = 2x + 2. It is given that c≠2c \ne 2.

  1. (a)

    Write down the value of dd and show that a=2a = 2 and b=4b = 4.

  2. (b)

    Hence find the range of values of cc if the graph has no stationary points.

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Worked solution

2(a)

1. Find the value of $d$ using the given vertical asymptote.

A vertical asymptote occurs when the denominator evaluates to zero (and the numerator is non-zero).
Given the vertical asymptote is x=−1x = -1, the denominator x+dx + d must be zero when x=−1x = -1.
−1+d=0  ⟹  d=1-1 + d = 0 \implies d = 1

2. Express the rational function in quotient-remainder form to identify its oblique asymptote.

Substitute d=1d = 1 into the function equation:
y=ax2+bx+cx+1y = \frac{ax^2 + bx + c}{x + 1}
By performing polynomial long division or expressing the numerator in terms of the denominator:
ax2+bx+c=ax(x+1)+(b−a)(x+1)+c−(b−a)ax^2 + bx + c = ax(x + 1) + (b - a)(x + 1) + c - (b - a)
y=ax+(b−a)+c−b+ax+1y = ax + (b - a) + \frac{c - b + a}{x + 1}
For large values of ∣x∣|x|, the fractional term c−b+ax+1\frac{c - b + a}{x + 1} approaches 00.
Therefore, the equation of the oblique asymptote is y=ax+(b−a)y = ax + (b - a).

3. Compare coefficients with the given oblique asymptote to show $a = 2$ and $b = 4$.

The given oblique asymptote is y=2x+2y = 2x + 2. Comparing this with y=ax+(b−a)y = ax + (b - a):
Comparing the coefficient of xx:
a=2a = 2
Comparing the constant term:
b−a=2b - a = 2
b−2=2  ⟹  b=4b - 2 = 2 \implies b = 4

Answer

d=1d = 1

2(b)

1. Substitute the values of $a$, $b$, and $d$ from part (a) into the function and differentiate with respect to $x$.

From part (a), a=2a = 2, b=4b = 4 and d=1d = 1, so the equation of the curve is
y=2x2+4x+cx+1y = \frac{2x^2 + 4x + c}{x+1}
Differentiating using the quotient rule:
dydx=(4x+4)(x+1)−(2x2+4x+c)(1)(x+1)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{(4x+4)(x+1) - (2x^2+4x+c)(1)}{(x+1)^2}
dydx=4x2+8x+4−2x2−4x−c(x+1)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{4x^2 + 8x + 4 - 2x^2 - 4x - c}{(x+1)^2}
dydx=2x2+4x+4−c(x+1)2\frac{\mathrm{d}y}{\mathrm{d}x} = \frac{2x^2 + 4x + 4 - c}{(x+1)^2}

2. Apply the condition for no stationary points by considering the discriminant of the numerator.

For stationary points, dydx=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0, which requires the numerator to be zero:
2x2+4x+(4−c)=02x^2 + 4x + (4 - c) = 0
For the graph to have no stationary points, this quadratic equation must have no real roots (since the denominator (x+1)2≠0(x+1)^2 \neq 0 for x≠−1x \neq -1). Thus, the discriminant must be strictly less than zero:
Δ<0\Delta < 0
(4)2−4(2)(4−c)<0(4)^2 - 4(2)(4 - c) < 0
16−32+8c<016 - 32 + 8c < 0
8c<168c < 16
c<2c < 2

Answer

c<2c < 2

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