Aspire Faculty ID #13745 · Topic: JAMIA MILLIA ISLAMIA MCA 2022 · Just now
JAMIA MILLIA ISLAMIA MCA 2022

If $z$ is any complex number with $|z-3-2i|\le2$, then the minimum of $|\,2z-6+5i\,|$ is:

Solution

Put $w=z-(3+2i)$ so $|w|\le2$. Then $2z-6+5i=2w+9i$. Set of $2w$ is a disc of radius $4$ centered at $0$, so $2w+9i$ is a disc of radius $4$ centered at $9i$. Minimum modulus $=\big||9|-4\big|=5$.

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