Lời giải:
a)Hinh tron co ban kinh 3cm thi chu vi la $2×3,14×3=18,84(cm)$
b)Hinh tron co duong kinh 5dm thi chu vi la $5×3,14=15,7(dm)$
c)Hinh thang co trung binh cong hai day 45,5cm va chieu cao 21cm thi dien tich la $\frac{45,5×2×21}{2}=955,5(cm^2)$