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The sum of the square of the modulus of the elements in the set $$\{z = a + ib : a, b \in \mathbb{Z}, z \in \mathbb{C}, |z - 1| \leq 1, |z - 5| \leq |z - 5i|\}$$ is ________
Correct Answer: 9
We need $$z = a + ib$$ with $$a, b \in \mathbb{Z}$$ satisfying: $$|z-1| \leq 1$$ and $$|z-5| \leq |z-5i|$$.
Condition 1: $$|z-1| \leq 1$$ means $$(a-1)^2 + b^2 \leq 1$$.
Since $$a, b$$ are integers: possible points are $$(0,0), (1,0), (2,0), (1,1), (1,-1)$$.
Check: $$(0,0)$$: $$(0-1)^2+0 = 1 \leq 1$$ âś“. $$(1,0)$$: $$0 \leq 1$$ âś“. $$(2,0)$$: $$1 \leq 1$$ âś“. $$(1,1)$$: $$0+1=1 \leq 1$$ âś“. $$(1,-1)$$: $$0+1=1 \leq 1$$ âś“.
Condition 2: $$|z-5| \leq |z-5i|$$, i.e., $$(a-5)^2+b^2 \leq a^2+(b-5)^2$$.
$$a^2-10a+25+b^2 \leq a^2+b^2-10b+25$$
$$-10a \leq -10b \implies b \leq a$$.
From the 5 points, those with $$b \leq a$$:
$$(0,0)$$: $$0 \leq 0$$ âś“. $$(1,0)$$: $$0 \leq 1$$ âś“. $$(2,0)$$: $$0 \leq 2$$ âś“. $$(1,1)$$: $$1 \leq 1$$ âś“. $$(1,-1)$$: $$-1 \leq 1$$ âś“.
All 5 points satisfy both conditions.
Sum of $$|z|^2$$: $$0 + 1 + 4 + 2 + 2 = 9$$.
The answer is $$\boxed{9}$$.
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