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6/7x5/8x7/3x7/6x8/5=\(\frac{6x5x7x7x8}{7x8x3x6x5}\)=gút gọn được \(\frac{7}{3}\)
\(\frac{6}{7}\times\frac{5}{8}\times\frac{7}{3}\times\frac{7}{6}\times\frac{8}{5}\)
\(=\frac{6\times5\times7\times7\times8}{7\times8\times3\times6\times5}\)
\(=\)6 x 5 x 7 x 7 x 8 / 7 x 8 x 3 x 6 x 5
\(=\frac{7}{3}\)
Mình sửa lại đề một chút nhé.
\(\dfrac{4}{3\times7}+\dfrac{4}{7\times11}+\dfrac{4}{11\times15}+\dfrac{4}{15\times19}+\dfrac{4}{19\times23}+\dfrac{4}{23\times27}\)
\(=\dfrac{1}{3}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{19}+\dfrac{1}{19}-\dfrac{1}{23}+\dfrac{1}{23}-\dfrac{1}{27}\)
\(=\dfrac{1}{3}-\dfrac{1}{27}\)
\(=\dfrac{8}{27}\).
A = \(\dfrac{4}{3\times7}\) + \(\dfrac{4}{7\times11}\) + \(\dfrac{4}{11\times15}\) + \(\dfrac{4}{15\times19}\) + \(\dfrac{4}{19\times23}\) + \(\dfrac{4}{23\times27}\)
A =1/3 -1/7+1/7-1/11 + 1/11-1/15 + 1/15 - 1/19 + 1/19 -1/23+1/23-1/27
A = 1/3 - 1/27
A = 8/27
a=\(1\times3+\dfrac{2}{3}\times7+\dfrac{3}{7}\times13+\dfrac{4}{13}\times21+\dfrac{5}{12}\times21\)
=3+\(\dfrac{14}{3}\)+\(\dfrac{39}{7}\)+\(\dfrac{84}{13}\)+5=8+\(\dfrac{215}{21}\)+\(\dfrac{84}{13}\)=\(\dfrac{4559}{273}\)+8=\(\dfrac{6743}{273}\)
=180x4x5/3x7
=720x5/3x7
=1200x7
=8400
\(36\) x \(5\)x\(4\)x\(\frac{5}{3}\)x\(7\)
\(=8400\)
Chúc em học tốt!!!!!!!!!!!!!