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The equation of a circle centered at point (x1,y1) with a radius r is given by

are you sure you wrote down the problem correctly?
Answer:
3/5
Step-by-step explanation:
12/20
(divide both by 2)
12/2= 6
20/2=10
6/10
(divide by both by 2)
6/2= 3
10/2= 5
3/5
The answer is, 4 + 8 + 12 + 16 ... rule is 4n.
Explanation:
one square has 4 toothpicks, then you added 4 picks (because you started with zero squares).
4 squares have 12 toothpicks, then you added 8 toothpicks to pass from one square to 4 squeres.
If you follow that reasoing you get to realize that the additions are 4, 8, 12, 16.