Giải Bài 1 trang 39 Vở thực hành Toán 5

Lời giải

Ta nhớ lại quan hệ giữa các đơn vị đo diện tích: hai đơn vị liền nhau hơn (kém) nhau 100 lần, nên $1\,\text{dm}^2=\dfrac{1}{100}\,\text{m}^2$ và $1\,\text{cm}^2=\dfrac{1}{100}\,\text{dm}^2$.

a) Viết dưới dạng $\text{m}^2$

- $8\,\text{m}^2\,75\,\text{dm}^2$: vì $75\,\text{dm}^2=\dfrac{75}{100}\,\text{m}^2=0{,}75\,\text{m}^2$ nên
$$8\,\text{m}^2\,75\,\text{dm}^2 = 8\dfrac{75}{100}\,\text{m}^2 = 8{,}75\,\text{m}^2.$$

- $3\,\text{m}^2\,6\,\text{dm}^2$: vì $6\,\text{dm}^2=\dfrac{6}{100}\,\text{m}^2=0{,}06\,\text{m}^2$ (hàng phần trăm) nên
$$3\,\text{m}^2\,6\,\text{dm}^2 = 3\dfrac{6}{100}\,\text{m}^2 = 3{,}06\,\text{m}^2.$$

- $120\,\text{dm}^2$: vì $120\,\text{dm}^2=\dfrac{120}{100}\,\text{m}^2$ nên
$$120\,\text{dm}^2 = 1{,}2\,\text{m}^2.$$

b) Viết dưới dạng $\text{dm}^2$

- $4\,\text{dm}^2\,25\,\text{cm}^2$: vì $25\,\text{cm}^2=\dfrac{25}{100}\,\text{dm}^2=0{,}25\,\text{dm}^2$ nên
$$4\,\text{dm}^2\,25\,\text{cm}^2 = 4\dfrac{25}{100}\,\text{dm}^2 = 4{,}25\,\text{dm}^2.$$

- $2\,\text{dm}^2\,5\,\text{cm}^2$: vì $5\,\text{cm}^2=\dfrac{5}{100}\,\text{dm}^2=0{,}05\,\text{dm}^2$ (hàng phần trăm) nên
$$2\,\text{dm}^2\,5\,\text{cm}^2 = 2\dfrac{5}{100}\,\text{dm}^2 = 2{,}05\,\text{dm}^2.$$

- $85\,\text{cm}^2$: vì $85\,\text{cm}^2=\dfrac{85}{100}\,\text{dm}^2$ nên
$$85\,\text{cm}^2 = 0{,}85\,\text{dm}^2.$$

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