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9709 P11 - Jun 2019 - Q2
318
The line \(4y = x + c\), where \(c\) is a constant, is a tangent to the curve \(y^2 = x + 3\) at the point \(P\) on the curve.
(i) Find the value of \(c\).
(ii) Find the coordinates of \(P\).
Solution
(i) To find the value of \(c\), we need the line \(4y = x + c\) to be tangent to the curve \(y^2 = x + 3\). Substitute \(x = 4y - c\) into the curve equation:
\(y^2 = 4y - c + 3\)
\(y^2 - 4y + c - 3 = 0\)
This is a quadratic in \(y\). For the line to be tangent, the discriminant must be zero:
\(b^2 - 4ac = 0\)
Here, \(a = 1\), \(b = -4\), \(c = c - 3\). So:
\((-4)^2 - 4(1)(c - 3) = 0\)
\(16 - 4c + 12 = 0\)
\(28 - 4c = 0\)
\(4c = 28\)
\(c = 7\)
(ii) With \(c = 7\), substitute back to find \(P\):