如图,有一圆弧形门拱的拱高AB为1m,跨度CD为4m,则这个圆弧形门拱的半径为 m.
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计算:x3· x —3 = ______;a 6÷a2·a3 =___________;2 0 + 2—1 =___ ___.
找规律填空: ……
若a+b="5," ab=6,则a2+b2=________