Find the values of pp for which the line y=x+1y = x + 1 is a tangent to the curve y=x2+(2p+3)x+p+4y = x^2 + (2p + 3)x + p + 4. With the values of pp found in part (i), find the coordinates of the points where the line meets the curve.

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Worked solution

7(i)

1. Equate the equations of the line and the curve to find their points of intersection.

x2+(2p+3)x+p+4=x+1x^2 + (2p + 3)x + p + 4 = x + 1

2. Rearrange the equation into the standard quadratic form $ax^2 + bx + c = 0$.

x2+(2p+3−1)x+p+4−1=0x^2 + (2p + 3 - 1)x + p + 4 - 1 = 0
x2+(2p+2)x+p+3=0x^2 + (2p + 2)x + p + 3 = 0

3. Apply the condition for a line to be a tangent to a curve (discriminant equals zero).

b2−4ac=0b^2 - 4ac = 0
(2p+2)2−4(1)(p+3)=0(2p + 2)^2 - 4(1)(p + 3) = 0

4. Solve the resulting quadratic equation for $p$.

4p2+8p+4−4p−12=04p^2 + 8p + 4 - 4p - 12 = 0
4p2+4p−8=04p^2 + 4p - 8 = 0
p2+p−2=0p^2 + p - 2 = 0
(p+2)(p−1)=0(p + 2)(p - 1) = 0
p=−2 or p=1p = -2 \text{ or } p = 1

Answer

p=−2p = -2 or p=1p = 1

7(ii)

1. State the values of $p$ from part (i) and the intersection equation

From part (i), the values of pp are p=−2p = -2 and p=1p = 1.
The intersection equation is x2+(2p+2)x+p+3=0x^2 + (2p + 2)x + p + 3 = 0.

2. Find the coordinates for $p = -2$

Substitute p=−2p = -2 into the intersection equation:
x2+(2(−2)+2)x+(−2)+3=0x^2 + (2(-2) + 2)x + (-2) + 3 = 0
x2−2x+1=0x^2 - 2x + 1 = 0
(x−1)2=0(x - 1)^2 = 0
x=1x = 1
Substitute x=1x = 1 into the line equation y=x+1y = x + 1:
y=1+1=2y = 1 + 1 = 2
The point is (1,2)(1, 2).

3. Find the coordinates for $p = 1$

Substitute p=1p = 1 into the intersection equation:
x2+(2(1)+2)x+1+3=0x^2 + (2(1) + 2)x + 1 + 3 = 0
x2+4x+4=0x^2 + 4x + 4 = 0
(x+2)2=0(x + 2)^2 = 0
x=−2x = -2
Substitute x=−2x = -2 into the line equation y=x+1y = x + 1:
y=−2+1=−1y = -2 + 1 = -1
The point is (−2,−1)(-2, -1).

Answer

(1,2)(1, 2) and (−2,−1)(-2, -1)

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