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June 2017 p32 q6
1968
The complex number \(2 - i\) is denoted by \(u\).
(i) It is given that \(u\) is a root of the equation \(x^3 + ax^2 - 3x + b = 0\), where the constants \(a\) and \(b\) are real. Find the values of \(a\) and \(b\).
(ii) On a sketch of an Argand diagram, shade the region whose points represent complex numbers \(z\) satisfying both the inequalities \(|z - u| < 1\) and \(|z| < |z + i|\).
Solution
(i) To find \(a\) and \(b\), substitute \(x = 2 - i\) into the equation \(x^3 + ax^2 - 3x + b = 0\). Calculate \((2-i)^3\) and \((2-i)^2\), and equate real and imaginary parts to zero.
Substitute into the equation: \((2 - 11i) + a(3 - 4i) - 3(2 - i) + b = 0\)
Equate real parts: \(2 + 3a - 6 + b = 0\)
Equate imaginary parts: \(-11 - 4a + 3 = 0\)
Solve these equations to find \(a = -2\) and \(b = 10\).
(ii) On the Argand diagram, draw a circle with center \(2 - i\) and radius 1. Also, draw the perpendicular bisector of the line segment joining \(0\) to \(-i\). Shade the region inside the circle and on the side of the bisector closer to \(0\).