\(A=\frac{3}{1.3}+\frac{3}{3.5}+.....+\frac{3}{19.21}\)
\(A=\frac{3}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+......+\frac{2}{19.21}\right)\)
\(A=\frac{3}{2}.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+......+\frac{1}{19}-\frac{1}{21}\right)\)
\(A=\frac{3}{2}.\left(1-\frac{1}{21}\right)\)
\(A=\frac{3}{2}.\frac{20}{21}\)
\(A=\frac{10}{7}\)
Ta có:
\(A=\frac{3}{1.3}+\frac{3}{3.5}+\frac{3}{5.7}+...+\frac{3}{19.21}\)
\(\Rightarrow A=\frac{2}{3}.\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+....+\frac{1}{19}-\frac{1}{21}\right)\)
\(\Rightarrow A=\frac{2}{3}.\left(\frac{1}{1}-\frac{1}{21}\right)=\frac{2}{3}.\frac{20}{21}=\frac{40}{63}\)
A = \(\frac{3}{1x3}+\frac{3}{3x5}+\frac{3}{5x7}+.......+\frac{3}{19x21}\)
A : 3 = \(\frac{1}{1x3}+\frac{1}{3x5}+\frac{1}{5x7}+.........+\frac{1}{19x21}\)
A : 3 = \(\frac{1}{1}+\frac{1}{3}-\frac{1}{3}+\frac{1}{5}-\frac{1}{5}+\frac{1}{7}-.........+\frac{1}{19}-\frac{1}{21}\)
=> A : 3 = \(\frac{1}{1}-\frac{1}{21}\)= \(\frac{21}{21}-\frac{1}{21}=\frac{20}{21}\)
=> A = \(\frac{20}{21}x3=\frac{60}{21}=\frac{20}{7}\)
\(A=\frac{3}{1.3}+\frac{3}{3.5}+\frac{3}{5.7}+...+\frac{3}{19.21}\)
\(A.3=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\)
\(A.3=1+\frac{1}{3}-\frac{1}{3}+\frac{1}{5}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{21}\)
\(A.3=1-\frac{1}{21}\)
\(A.3=\frac{20}{21}\)
\(A=\frac{20}{63}\)
Cô giáo mk dạy đó
\(A=\frac{3}{1.3}+\frac{3}{3.5}+\frac{3}{5.7}+...+\frac{3}{19.21}\)
\(A=\left(\frac{3}{1}-\frac{3}{3}\right):2+\left(\frac{3}{3}-\frac{3}{5}\right):2+...+\left(\frac{3}{19}-\frac{3}{21}\right):2\)
\(A=\frac{10}{7}\)
Bạn thêm vào sau dòng 2 nhé :
\(A=\left(\frac{3}{1}-\frac{3}{3}+\frac{3}{3}-\frac{3}{5}+...+\frac{3}{19}-\frac{3}{21}\right):2\)
\(A=\left(\frac{3}{1}-\frac{3}{21}\right):2\)
