You can
put this solution on YOUR website! .
This regular octagon has the central angle of
= 45 degrees
and has the side length of 7*2*sin(22.5°) = 7*2*0.3827 = 5.3578 centimeters,
NOT 4 centimeters.
So, your input information is WRONG : it is INCORRECT.
Such regular octagon as described in your post DOES NOT EXIST and CAN NOT EXIST.
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Please do not create F A K E problems and do not post F A K E problems to this forum.
T H A N K S .