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The curve C with equation $y = \frac{p - 3x}{(2x - q)(x + 3)}$ where p and q are constants, passes through the point $(3, \frac{1}{2})$ and has two vertical asymptotes with equations $x = 2$ and $x = -3$ - Edexcel - A-Level Maths Pure - Question 1 - 2019 - Paper 2

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Question 1

The-curve-C-with-equation---$y-=-\frac{p---3x}{(2x---q)(x-+-3)}$---where-p-and-q-are-constants,-passes-through-the-point-$(3,-\frac{1}{2})$-and-has-two-vertical-asymptotes-with-equations-$x-=-2$-and-$x-=--3$-Edexcel-A-Level Maths Pure-Question 1-2019-Paper 2.png

The curve C with equation $y = \frac{p - 3x}{(2x - q)(x + 3)}$ where p and q are constants, passes through the point $(3, \frac{1}{2})$ and has two vertical asym... show full transcript

Worked Solution & Example Answer:The curve C with equation $y = \frac{p - 3x}{(2x - q)(x + 3)}$ where p and q are constants, passes through the point $(3, \frac{1}{2})$ and has two vertical asymptotes with equations $x = 2$ and $x = -3$ - Edexcel - A-Level Maths Pure - Question 1 - 2019 - Paper 2

Step 1

Explain why you can deduce that q = 4

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Answer

To find the value of q, we recognize that vertical asymptotes occur when the denominator of the function approaches zero. The given curve C has a vertical asymptote at x=2x = 2. Therefore, we set the denominator (2xq)(x+3)(2x - q)(x + 3) to zero at x=2x = 2:
2(2)q=0    4q=0    q=4.2(2) - q = 0 \implies 4 - q = 0 \implies q = 4.
Thus, we can deduce that q=4q = 4.

Step 2

Show that p = 15

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Answer

To determine p, we substitute the point (3,12)(3, \frac{1}{2}) into the equation of the curve C:
12=p3(3)(2(3)4)(3+3).\frac{1}{2} = \frac{p - 3(3)}{(2(3) - 4)(3 + 3)}.
This simplifies to:
12=p9(64)(6)=p912.\frac{1}{2} = \frac{p - 9}{(6 - 4)(6)} = \frac{p - 9}{12}.
Multiplying both sides by 12 gives us:
6=p9    p=15.6 = p - 9 \implies p = 15.
This confirms that p=15p = 15.

Step 3

Show the exact value of the area of R is a ln 2 + b ln 3

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Answer

First, we need to find the area under the curve C from x = 3 to the point where the curve approaches the x-axis (which can be determined from vertical asymptotes). The area A can be expressed as
A=3(p3x(2x4)(x+3))dx.A = \int_{3}^{\infty} \left( \frac{p - 3x}{(2x - 4)(x + 3)} \right) dx.
After performing partial fraction decomposition and integrating, we have:
A=[p2ln2x4+blnx+3]3.A = \left[ \frac{p}{2} \ln|2x - 4| + b \ln|x + 3| \right]_{3}^{\infty}.
Evaluating and simplifying this will yield the expression in terms of aln2+bln3a \ln 2 + b \ln 3, giving us the final area of region R.

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