As shown in the figure, b=4-r can be obtained.
According to Pythagorean theorem, a? =r? -2? =r? -4
Because b=a? /4,
So 4-r=a? /4=r? /4- 1
The solution is r = 2 √ 6-2.
Discard negative roots, with a maximum radius of 2√6-2.