The diagram shows the curve y = x^2 + px + q. The points A(0, −5) and B(2, −9) lie on the graph. B(2, −9) is the minimum point.

  1. (a)

    Find the value of p and of q.

    Answer p =

    q =

  2. (b)

    Find the equation of the straight line that is parallel to line BC and passes through A, if the coordinates of C is (7,16).

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Worked solution

16(a)

1. Use point A to find $q$.

Since A(0,−5)A(0,-5) lies on the curve,
−5=02+p(0)+q-5=0^2+p(0)+q
q=−5q=-5

2. Use point B to form an equation involving $p$.

Since B(2,−9)B(2,-9) lies on the curve,
−9=22+p(2)+q-9=2^2+p(2)+q
−9=4+2p+q-9=4+2p+q

3. Substitute $q=-5$ and solve for $p$.

Substitute q=−5q=-5:
−9=4+2p−5-9=4+2p-5
−9=2p−1-9=2p-1
2p=−82p=-8
p=−4p=-4

Answer

p=−4p=-4
q=−5q=-5

16(b)

1. Find the gradient of line $BC$.

For B(2,−9)B(2,-9) and C(7,16)C(7,16),
mBC=16−(−9)7−2=255=5.m_{BC}=\frac{16-(-9)}{7-2}=\frac{25}{5}=5.

2. Use the same gradient and point $A$ to find the required equation.

The required line is parallel to BCBC, so its gradient is also 55.

It passes through A(0,−5)A(0,-5), so the yy-intercept is −5-5.

Hence,
y=5x−5.y=5x-5.

Answer

y=5x−5y=5x-5

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