Question #226858
given that p and q are real & that 1+2i are aroot of the equation z^2+(p+5i)z +q(2-i) determine the value of p& q and the other root of the equation
1
Expert's answer
2021-08-18T11:53:42-0400

Given that pp and qq are real and that 1+2i1+2i are a root of the equation z2+(p+5i)z+q(2i)z^2+(p+5i)z +q(2-i) let us determine the value of pp and qq and the other root of the equation.

Let a+bia+bi be the other root of the equation. Then


z2+(p+5i)z+q(2i)=(z12i)(zabi)=z2+(12iabi)z+(1+2i)(a+bi).z^2+(p+5i)z +q(2-i)=(z-1-2i)(z-a-bi)\\ =z^2+(-1-2i-a-bi)z+(1+2i)(a+bi).


It follows that

p+5i=12iabi andq(2i)=(1+2i)(a+bi)=(a2b)+(2a+b)i.p+5i=-1-2i-a-bi\text{ and}\\ q(2-i)=(1+2i)(a+bi)=(a-2b)+(2a+b)i.

Therefore, we get the following system:


{p=1a5=2b2q=a2bq=2a+b\begin{cases} p=-1-a\\ 5=-2-b\\ 2q=a-2b\\ -q=2a+b \end{cases}


which is equivalent to the following systems


{p=1ab=72q=a+14q=2a7\begin{cases} p=-1-a\\ b=-7\\ 2q=a+14\\ -q=2a-7 \end{cases}


| let us myltiply the last equation by 2 and add to the third equation |


{p=1ab=72q=a+140=5a\begin{cases} p=-1-a\\ b=-7\\ 2q=a+14\\ 0=5a \end{cases}


{p=1b=7q=7a=0\begin{cases} p=-1\\ b=-7\\ q=7\\ a=0 \end{cases}


We conclude that p=1, q=7p=-1,\ q=7 and the other root of the equation is 7i.-7i.



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