Math Problem Statement

The figure shows the graph of y = -x^2 + 5x + p. The graph cuts the negative y-axis at a point, and its vertex lies above the x-axis. Find the range of the values of p.

Solution

Let's solve the problem step by step.

The given quadratic equation is:
y=x2+5x+py = -x^2 + 5x + p

Key observations:

  1. The graph is a parabola that opens downward because the coefficient of x2x^2 is negative (1-1).
  2. The vertex of the parabola is above the xx-axis.
  3. The graph intersects the yy-axis at a negative point (y<0y < 0).

We need to find the range of values of pp.


Step 1: Vertex of the parabola

The xx-coordinate of the vertex of a parabola y=ax2+bx+cy = ax^2 + bx + c is given by:
xv=b2ax_v = -\frac{b}{2a}
Here, a=1a = -1 and b=5b = 5. So:
xv=52(1)=52x_v = -\frac{5}{2(-1)} = \frac{5}{2}

Substitute xv=52x_v = \frac{5}{2} into the equation y=x2+5x+py = -x^2 + 5x + p to find the yy-coordinate of the vertex: yv=(52)2+5(52)+py_v = -\left(\frac{5}{2}\right)^2 + 5\left(\frac{5}{2}\right) + p Simplify: yv=254+252+py_v = -\frac{25}{4} + \frac{25}{2} + p Convert 252\frac{25}{2} to a denominator of 4: yv=254+504+p=254+py_v = -\frac{25}{4} + \frac{50}{4} + p = \frac{25}{4} + p

Since the vertex lies above the xx-axis (yv>0y_v > 0): 254+p>0\frac{25}{4} + p > 0 Solve for pp: p>254p > -\frac{25}{4}


Step 2: Intersecting the yy-axis

The graph cuts the yy-axis when x=0x = 0. Substituting x=0x = 0 into y=x2+5x+py = -x^2 + 5x + p:
y=py = p Since the graph intersects the yy-axis at a negative point: p<0p < 0


Step 3: Combine the conditions

From the vertex condition, p>254p > -\frac{25}{4}.
From the yy-axis condition, p<0p < 0.

Thus, the range of pp is: 254<p<0-\frac{25}{4} < p < 0


Final Answer:

The range of pp is: 254<p<0-\frac{25}{4} < p < 0


Would you like a detailed explanation for any part of this solution?

Here are some related questions to consider:

  1. How do we determine the vertex of a parabola?
  2. What does the condition p<0p < 0 imply about the graph?
  3. How do we know the graph opens downward?
  4. How would the solution change if the vertex were below the xx-axis?
  5. How can we verify the graph satisfies the given conditions for specific values of pp?

Tip: Always start analyzing quadratic equations by identifying the vertex and axis of symmetry.

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Math Problem Analysis

Mathematical Concepts

Quadratic Equations
Parabolas
Graph Analysis

Formulas

Vertex formula for a parabola: x_v = -b / (2a)
General quadratic equation: y = ax^2 + bx + c

Theorems

Properties of quadratic functions
Condition for the vertex above the x-axis (y_v > 0)

Suitable Grade Level

Grades 10-12