The line of symmetry of a quadratic curve is x=−2x = -2 and the curve lies above the xx-axis for all xx. Given that the point (−1,4)(-1, 4) lies on the curve, find a possible equation of the curve in the form y=a(x−h)2+ky = a(x - h)^2 + k where aa, hh, and kk are integers.

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Worked solution

4

1. Identify the value of $h$ from the line of symmetry.

The line of symmetry is x=−2x = -2, so h=−2h = -2.
The equation is of the form y=a(x+2)2+ky = a(x + 2)^2 + k.

2. Substitute the given point to find a relationship between $a$ and $k$.

Substitute x=−1x = -1 and y=4y = 4 into the equation:
4=a(−1+2)2+k4 = a(-1 + 2)^2 + k
4=a(1)2+k4 = a(1)^2 + k
a+k=4a + k = 4

3. Use the condition that the curve is always above the $x$-axis to restrict $a$ and $k$.

Since the curve lies above the xx-axis for all xx, it must open upwards (a>0a > 0) and its minimum value must be positive (k>0k > 0).
Given aa and kk are integers, and a+k=4a + k = 4, the possible integer pairs for (a,k)(a, k) are (1,3)(1, 3), (2,2)(2, 2), and (3,1)(3, 1).

4. State a possible equation.

Choosing a=1a = 1 and k=3k = 3:
y=(x+2)2+3y = (x + 2)^2 + 3

Answer

y=(x+2)2+3y = (x + 2)^2 + 3 (or y=2(x+2)2+2y = 2(x + 2)^2 + 2, or y=3(x+2)2+1y = 3(x + 2)^2 + 1)

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