hộ mới

tui mới lớp 4

25%. x + x= 1,25
25/100. x + x= 1,25
1/4. x + x= 125/100
x. ( 1/4 + 1) = 125/100
x . 5/4 = 5/4
x = 5/4 : 5/4
x = 1
Vậy x=1

Đặt tổng trên là A

\(\dfrac{2A}{7}=\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{2021.2023}=\)

\(=\dfrac{3-1}{1.3}+\dfrac{5-3}{3.5}+\dfrac{7-5}{5.7}+...+\dfrac{2023-2021}{2021.2023}=\)

\(=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{2021}-\dfrac{1}{2023}=\)

\(=\dfrac{2022}{2023}\Rightarrow A=\dfrac{7.2022}{2.2023}\)

\(A=\dfrac{7}{1.3}+\dfrac{7}{3.5}+\dfrac{7}{5.7}+...+\dfrac{7}{2021.2023}\\ \Rightarrow\dfrac{2}{7}A=\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{2021.2023}\\ \Rightarrow\dfrac{2}{7}A=2-\dfrac{2}{3}+\dfrac{2}{3}-\dfrac{2}{5}+\dfrac{2}{5}-\dfrac{2}{7}+...+\dfrac{2}{2021}-\dfrac{2}{2023}\\ \Rightarrow\dfrac{2}{7}A=2-\dfrac{2}{2023}=\dfrac{4044}{2023}\Rightarrow A=\dfrac{2022}{289}\)

a) đến đơn  vị: \(-457,1631\approx-460\) ; \(641,8491\approx642\)

b) đến hàng chục: \(-457,1631\approx-460\) ; \(641,8491\approx641\)

a) -457,1631 ≈ -457; 641,8941 ≈ 642

b) -457,1631 ≈ -458; 641,8491 ≈ 641

\(=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+.....+\dfrac{1}{18.19}+\dfrac{1}{19.20}\)

\(=\dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+....+\dfrac{1}{18}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{20}\)

\(=\dfrac{1}{1}-\dfrac{1}{20}=\dfrac{20}{20}-\dfrac{1}{20}\)

\(=\dfrac{19}{20}\).

NẾU CÓ GÌ SAI SÓT MONG BẠN THÔNG CẢM !!!

Bác nông dân đã mang số cam ra chợ là: \(30:\dfrac{2}{3}=45\left(kg\right)\)