In the xy𝑥𝑦-plane, a parabola has vertex (−25, 78)(-25, 78) and intersects the x𝑥-axis at two points. If the equation of the parabola is written in the form y=ax2+bx+c𝑦=𝑎𝑥2+𝑏𝑥+𝑐, where a𝑎, b𝑏, and c𝑐 are constants, which of the following could be the value of a+b+c𝑎+𝑏+𝑐?
Question
In the xy𝑥𝑦-plane, a parabola has vertex (−25, 78)(-25, 78) and intersects the x𝑥-axis at two points. If the equation of the parabola is written in the form y=ax2+bx+c𝑦=𝑎𝑥2+𝑏𝑥+𝑐, where a𝑎, b𝑏, and c𝑐 are constants, which of the following could be the value of a+b+c𝑎+𝑏+𝑐?
Solution
The vertex form of a parabola is given by y = a(x-h)² + k, where (h, k) is the vertex of the parabola. Given that the vertex is (-25, 78), we can substitute these values into the equation to get y = a(x + 25)² + 78.
The parabola also intersects the x-axis at two points. This means that the y-coordinate at these points is 0. So, we can set y = 0 in the equation and solve for x to find the x-intercepts.
0 = a(x + 25)² + 78 => a(x + 25)² = -78 => x = ± sqrt(-78/a) - 25
Since the parabola intersects the x-axis at two points, there are two solutions for x. These solutions are the roots of the quadratic equation ax² + bx + c = 0. By comparing this equation with the standard form of a quadratic equation, we can see that the sum of the roots is given by -b/a and the product of the roots is given by c/a.
Therefore, the sum of the constants a, b, and c is given by a + (-b/a) + (c/a) = a - b/a + c/a = a + c - b. Since the roots are symmetric about the vertex, the sum of the roots is 2h = 2*(-25) = -50. Therefore, a + c - b = -50.
So, the possible value of a + b + c is -50.
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