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Nov 2021 p31 q10
1936
The complex number 1 + 2i is denoted by u. The polynomial 2x^3 + ax^2 + 4x + b, where a and b are real constants, is denoted by p(x). It is given that u is a root of the equation p(x) = 0.
(a) Find the values of a and b.
(b) State a second complex root of this equation.
(c) Find the real factors of p(x).
(d) (i) On a sketch of an Argand diagram, shade the region whose points represent complex numbers z satisfying the inequalities |z - u| ≤ √5 and arg z ≤ 1/4 π.
(ii) Find the least value of Im z for points in the shaded region. Give your answer in an exact form.
Solution
(a) Substitute u = 1 + 2i into the polynomial p(x) = 2x^3 + ax^2 + 4x + b. Calculate u^2 = -3 + 4i and u^3 = -11 - 2i. Substitute these into the polynomial and equate real and imaginary parts to zero:
\(-18 - 3a + b = 0\)
\(4 + 4a = 0\)
\(Solving these equations gives a = -1 and b = 15.\)
(b) Since the coefficients are real, the complex roots occur in conjugate pairs. Therefore, the second root is 1 - 2i.
(c) The polynomial can be factored as (x^2 - 2x + 5)(2x + 3).
(d)(i) The region is a circle centered at 1 + 2i with radius √5, intersecting the line y = x in the first quadrant. Shade the region satisfying |z - u| ≤ √5 and arg z ≤ 1/4 π.