S=$\frac{5^{2}}{1X6}$ + $\frac{5^{2}}{6X11}$ + $\frac{5^{2}}{11X16}$ + $\frac{5^{2}}{16X21}$ + $\frac{5^{2}}{21X26}$
S=$\frac{5^{2}}{1X6}$ + $\frac{5^{2}}{6X11}$ + $\frac{5^{2}}{11X16}$ + $\frac{5^{2}}{16X21}$ + $\frac{5^{2}}{21X26}$
By Aubrey
By Aubrey
S=$\frac{5^{2}}{1X6}$ + $\frac{5^{2}}{6X11}$ + $\frac{5^{2}}{11X16}$ + $\frac{5^{2}}{16X21}$ + $\frac{5^{2}}{21X26}$
Đáp án:
S = 5.( $\frac{5}{1 x 6}$+$\frac{5}{6 x 11}$+$\frac{5}{11 x 16}$+$\frac{5}{16 x 21}$+$\frac{5}{21 x 26}$ )
= 5.( 1-$\frac{1}{6}$+$\frac{1}{6}$-$\frac{1}{11}$+$\frac{1}{11}$-$\frac{1}{16}$+$\frac{1}{21}$-$\frac{1}{21}$+$\frac{1}{21}$-$\frac{1}{26}$
= 5. ( 1 – $\frac{1}{26}$ )
= 5. $\frac{25}{26}$
= $\frac{125}{26}$
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