Giải bài 2 trang 30 - SGK Toán lớp 5

Ta có:

$\begin{align} & 2{{m}^{2}}9\text{d}{{m}^{2}}=200d{{m}^{2}}+9\text{d}{{m}^{2}}=209\text{d}{{m}^{2}} \\ &\text{Vì} \, 209\text{d}{{m}^{2}}\,>\,29\text{d}{{m}^{2}}\,\, \\ & \Rightarrow \,2{{m}^{2}}9\text{d}{{m}^{2}}\,>\,29\text{d}{{m}^{2}} \\ \end{align}$

$\begin{align} & 79k{{m}^{2}}=7900ha \\ & \text{Vì} \, 790ha\,<\,7900ha \\ & \Rightarrow \,790ha\,\,<\,\,79k{{m}^{2}} \\ \end{align}$

$\begin{align} & 8\text{d}{{m}^{2}}5c{{m}^{2}}=800c{{m}^{2}}+5c{{m}^{2}}=805c{{m}^{2}} \\ & \text{Vì} \, 805c{{m}^{2}}\,\,<\,\,810c{{m}^{2}} \\ & \Rightarrow \,8\text{d}{{m}^{2}}5c{{m}^{2}}\,\,<\,\,810c{{m}^{2}} \\ \end{align}$

$\begin{align} & 4c{{m}^{2}}5m{{m}^{2}}=4\frac{5}{100}c{{m}^{2}} \\ & \text{Vì} \, 4\frac{5}{100}c{{m}^{2}}=4\frac{5}{100}c{{m}^{2}} \\ & \Rightarrow \,\,4c{{m}^{2}}5m{{m}^{2}}=4\frac{5}{100}c{{m}^{2}} \\ \end{align}$

Lưu ý: Ta đổi ra cùng đơn vị đo diện tích rồi mới so sánh