The complex number $$\(1+2 \mathrm{i}\)$$ is denoted by $$\(u\)$$. The polynomial $$\(2 x^{3}+a x^{2}+4 x+b\)$$, where $$\(a\)$$ and $$\(b\)$$ are real constants, is denoted by $$\(\mathrm{p}(x)\)$$. It is given that $$\(u\)$$ is a root of the equation $$\(\mathrm{p}(x)=0\)$$. Find the values of $$\(a\)$$ and $$\(b\)$$. ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................

Mathematics
IGCSE&ALevel
CAIE
Exam No:9709_w21_qp_31 Year:2021 Question No:10(a)

Answer:

Substitute \(1+2 i\) in the polynomial and attempt expansions of \(x^{2}\) and \(x^{3}\)
Equate real and/or imaginary parts to zero
Obtain \(a=-1\)
Obtain \(b=15\)

Knowledge points:

3.9.1 understand the idea of a complex number, recall the meaning of the terms real part, imaginary part, modulus, argument, conjugate, and use the fact that two complex numbers are equal if and only if both real and imaginary parts are equal
3.9.2 carry out operations of addition, subtraction, multiplication and division of two complex numbers expressed in Cartesian form (For calculations involving multiplication or division, full details of the working should be shown.)

Solution:

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