3A=5(3/1*4+3/4*7+...+3/91*94)
3A=5(1-1/4+1/4-1/7+...+1/91-1/94)
3A=5(1-1/94)
3A=5.93/94
A=(5.93/94):3
A=155/94
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Ta đặt
\(A=\dfrac{5}{1\times4}+\dfrac{5}{4\times7}+...+\dfrac{5}{91\times94}\)
\(\dfrac{3}{5}A=\dfrac{3}{1\times4}+\dfrac{3}{4\times7}+....+\dfrac{3}{91\times94}\)
\(\dfrac{3}{5}A=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{91}+\dfrac{1}{94}\)
\(\dfrac{3}{5}A=1-\dfrac{1}{94}=\dfrac{93}{94}\)
\(A=\dfrac{93}{94}\div\dfrac{3}{5}=\dfrac{155}{94}\)
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