Author Topic: Q7 TUT 0203  (Read 448 times)

Victor Ivrii

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Q7 TUT 0203
« on: November 30, 2018, 03:57:02 PM »
Using argument principle along line on the picture, calculate the number of zeroes of the following function in the given annulus:
$$
4z^3- 12z^2 + 2z + 10 \qquad \text{in  }\ \bigl\{\frac{1}{2}< |z| < 2\bigr\}.
$$

Siying Li

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Re: Q7 TUT 0203
« Reply #1 on: November 30, 2018, 04:13:06 PM »
Let$f\left(z\right)\mathrm{=4}z^{\mathrm{3}}\mathrm{-}\mathrm{12}z^{\mathrm{2}}\mathrm{+2}z\mathrm{+10}$

Since $\frac{\mathrm{1}}{\mathrm{2}}\mathrm{<}\left|z\right|\mathrm{<2}$

When$\mathrm{\ }\left|z\right|\mathrm{=}\frac{\mathrm{1}}{\mathrm{2}}$
$\left|\mathrm{4}z^{\mathrm{3}}\mathrm{-}\mathrm{12}z^{\mathrm{2}}\right|\mathrm{=}\left|\mathrm{4}\right|\left|z\right|^{\mathrm{3}}\mathrm{-}\mathrm{12\times }\left|z\right|^{\mathrm{2}}\mathrm{=}\frac{\mathrm{5}}{\mathrm{2}}\mathrm{<}\left|\mathrm{2}\right|\left|\mathrm{z}\right|\mathrm{+10}\mathrm{=}\mathrm{2\times }\frac{\mathrm{1}}{\mathrm{2}}\mathrm{+10}\mathrm{=11}\\
\mathrm{2}z\mathrm{+10=0}\mathrm{\Rightarrow }z\mathrm{=-5}\\
\left|\mathrm{-}\mathrm{5}\right|\mathrm{>}\left|\frac{\mathrm{1}}{\mathrm{2}}\right|\\$
Then $\mathrm{2}z\mathrm{+10}$ has no zero in $\left|z\right|\mathrm{<}\frac{\mathrm{1}}{\mathrm{2}}$

When$\mathrm{\ }\left|z\right|\mathrm{=2}$
$\left|\mathrm{2}z\mathrm{+10}\right|\mathrm{=}\left|\mathrm{2}\right|\left|z\right|+10\mathrm{=14<}\left|\mathrm{4}\right|\left|\mathrm{z}\right|^{\mathrm{3}}\mathrm{-}\mathrm{12}\left|\mathrm{z}\right|^{\mathrm{2}}\mathrm{=}\mathrm{4\times }{\mathrm{2}}^{\mathrm{3}}\mathrm{-}\mathrm{12\times }{\mathrm{2}}^{\mathrm{2}}\mathrm{=16}\\
\mathrm{4}z^{\mathrm{3}}\mathrm{-}\mathrm{12}z^{\mathrm{2}}\mathrm{=0}\\
\mathrm{4}z^{\mathrm{2}}\left(z\mathrm{-}\mathrm{3}\right)\mathrm{=0}\mathrm{\Rightarrow }\mathrm{4}z^{\mathrm{2}}\mathrm{=0,}z\mathrm{=3}\\
\left|\mathrm{3}\right|\mathrm{>}\left|\mathrm{2}\right|\\ $
Then $\mathrm{4}z^{\mathrm{3}}\mathrm{-}\mathrm{12}z^{\mathrm{2}}$ has 2 zeros in $\left|z\right|\mathrm{<2}$

Then number of zeros of f(x) in the region is 2 - 0 = 2
« Last Edit: November 30, 2018, 08:45:26 PM by Siying Li »