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perfectly round; perfectly circular
まん円い顔をした赤ちゃんが笑っていた。
A baby with a perfectly round face was smiling.
池に落ちた水滴の波紋がまん円かった。
The ripples from the water drop that fell into the pond were perfectly circular.
急いで丸めたので、おにぎりがまん円くない。
I shaped it in a hurry, so the rice ball isn't perfectly round.