Porisma

Worked solution

Recover a vertical hyperbola from vertices and asymptotes

Find the equation of the hyperbola shown below.

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Centre(−1,2), vertices (−1,5),(−1,−1), asymptotes y=(3/4)x+11/4 and y=−(3/4)x+5/4. Find the hyperbola’s equation.
Centre(−1,2), vertices (−1,5),(−1,−1), asymptotes y=(3/4)x+11/4 and y=−(3/4)x+5/4. Find the hyperbola’s equation.
Centre(−1,2), vertices (−1,5),(−1,−1), asymptotes y=(3/4)x+11/4 and y=−(3/4)x+5/4. Find the hyperbola’s equation.

CJC · Lecture · Example 6

Read the centre and vertical size

Use the vertical form (y−k)2/b2−(x−h)2/a2=1(y-k)^2/b^2-(x-h)^2/a^2=1. The centre is (−1,2)(-1,2), so h=−1,k=2h=-1,k=2. The vertices (−1,5),(−1,−1)(-1,5),(-1,-1) are each 33 units vertically from the centre, so b=3b=3. This is the Teacher copy’s alternative-method note: the given vertex ordinates fix bb.

How did this part go?

Use the asymptote slopes

The given lines y=34x+114y=\tfrac34x+\tfrac{11}{4} and y=−34x+54y=-\tfrac34x+\tfrac54 rewrite as y=2±34(x+1)y=2\pm\tfrac34(x+1). Compare with y=k±(b/a)(x−h)y=k\pm(b/a)(x-h): b/a=3/4b/a=3/4. Together with b=3b=3, this gives a=4a=4. The ratio alone does not fix the sizes of aa and bb; the vertex distance supplies the missing scale.

How did this part go?

Write and check the equation

The equation is (y−2)232−(x+1)242=1\dfrac{(y-2)^2}{3^2}-\dfrac{(x+1)^2}{4^2}=1. It gives vertices (−1,2±3)(-1,2\pm3) and asymptote slopes ±3/4\pm3/4 through (−1,2)(-1,2), matching all the diagram labels.

How did this part go?

Common mistakes

Asymptote slopes determine b/a, not the individual values. Use a vertex to determine the scale.

Solution notes

Student/Teacher PDF 22, printed 134. Teacher alternative note b=3 integrated into the first working step. No marks printed.

The source’s short transition from the ratio to a=4,b=3 uses the plotted vertices as well as the slopes; this dependency is now explicit.

Read the hint