IB Mathematics SL 2.6 The quadratic function AA SL Paper 1- Exam Style Questions- New Syllabus
The following diagram shows part of the graph of a quadratic function \( f \). The vertex of the parabola is \((-2, -5)\) and the \( y \)-intercept is at point \( P \).
Part (a):
Write down the equation of the axis of symmetry.
Part (b):
The function can be written in the form \( f(x) = \frac{1}{4}(x – h)^2 + k \), where \( h, k \in \mathbb{Z} \). Write down the values of \( h \) and \( k \).
Part (c):
Find the \( y \)-coordinate of \( P \). (Line \( L \) is normal to \( f \) at point \( P \).
Part (d):
Find the equation of the line \( L \), in the form \( y = ax + b \).
Part (e):
Calculate the distance between \( P \) and \( Q \).
▶️ Answer/Explanation
Part (a)
The vertex of the parabola is at \((-2, -5)\). The axis of symmetry is the vertical line passing through the vertex.
Answer: \( x = -2 \)
Part (b)
The function is given in the form \( f(x) = \frac{1}{4}(x – h)^2 + k \). The vertex is at \((-2, -5)\), so \( h \) and \( k \) correspond to the x- and y-coordinates of the vertex.
Answer: \( h = -2 \), \( k = -5 \)
Part (c)
The \( y \)-intercept occurs at \( x = 0 \). Using the function \( f(x) = \frac{1}{4}(x + 2)^2 – 5 \):
\[ f(0) = \frac{1}{4}(0 + 2)^2 – 5 = \frac{1}{4} \cdot 4 – 5 = 1 – 5 = -4 \]
Answer: The \( y \)-coordinate of \( P \) is \( -4 \).
Part (d)
Line \( L \) is normal to \( f(x) \) at \( P(0, -4) \). First, find the slope of the tangent at \( x = 0 \):
\[ f(x) = \frac{1}{4}(x + 2)^2 – 5 \]
\[ f'(x) = \frac{1}{4} \cdot 2(x + 2) = \frac{1}{2}(x + 2) \]
\[ f'(0) = \frac{1}{2}(0 + 2) = 1 \]
The slope of the normal is the negative reciprocal: \( m = -\frac{1}{1} = -1 \).
Using point-slope form at \( P(0, -4) \):
\[ y – (-4) = -1(x – 0) \]
\[ y + 4 = -x \implies y = -x – 4 \]
Answer: \( y = -x – 4 \)
Part (e)
Find point \( Q \), the intersection of \( f(x) \) and line \( L \):
\[ f(x) = \frac{1}{4}(x + 2)^2 – 5 \]
\[ L: y = -x – 4 \]
Set \( f(x) = L \):
\[ \frac{1}{4}(x + 2)^2 – 5 = -x – 4 \]
Multiply by 4:
\[ (x + 2)^2 – 20 = -4x – 16 \]
\[ x^2 + 4x + 4 – 20 + 4x + 16 = 0 \]
\[ x^2 + 8x = 0 \]
\[ x(x + 8) = 0 \implies x = 0 \text{ or } x = -8 \]
\( x = 0 \) corresponds to \( P \), so \( Q \) is at \( x = -8 \).
Substitute \( x = -8 \) into \( L \):
\[ y = -(-8) – 4 = 8 – 4 = 4 \]
Thus, \( Q(-8, 4) \).
Calculate the distance between \( P(0, -4) \) and \( Q(-8, 4) \):
\[ \text{Distance} = \sqrt{(-8 – 0)^2 + (4 – (-4))^2} = \sqrt{64 + 64} = \sqrt{128} = 8\sqrt{2} \]
Answer: \( 8\sqrt{2} \)