如图，已知长方形的面积为12平方厘米，求半圆面积．
+ + ++ ﹣ ﹣++ ﹣﹣ 1﹣++﹣．
计算：= 1﹣= = = == = 1﹣= = =
+=﹣==改正：
用推算计算（1）+=（2）﹣=
计算：2.5×40= 18×6= 1.73+2.07= 10﹣0.9= 400÷4=+= ﹣= += ﹣= ﹣=
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