2 x ( 1/1x3 + 1/3x5 + 1/5x7 + ... + 1/13x15 )
= 2/1x3 + 2/3x5 + ... + 2/13x15
= 1 - 1/3 + 1/3 - 1/5 + ... + 1/13 - 1/15
= 1 - 1/15
= 14/15
Mk ko bt đúng hay sai
Tk mk nha
2.(1/1.3+1/3.5+1/5.7+...+1/13.15)
=2.1/1.3+2.1/3.5+......+2.1/13.15
=2/1.3+2/3.5+...+2/13.15
=1-1/3+1/3-1/5+...+1/13-1/15
=1-(1/3-1/3)-....-(1/13-1/13)-1/15
=1-1/15
=14/15
Trả lời :
\(2.\left(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{13.15}\right)\)
\(=\frac{2.1}{1.3}+\frac{2.1}{3.5}+\frac{2.1}{5.7}+...+\frac{2.1}{13.15}\)
\(=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{13.15}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-...+\frac{1}{13}-\frac{1}{15}\)
\(=1-\frac{1}{15}\)
\(=\frac{14}{15}\)
\(\downarrow\)Tk nha , thanks
\(2\times\left(\frac{1}{1\times3}+\frac{1}{3\times5}+\frac{1}{5\times7}+...+\frac{1}{13\times15}\right)\)
\(=\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{13\times15}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{13}-\frac{1}{15}\)
\(=1+\left(\frac{1}{3}-\frac{1}{3}\right)+\left(\frac{1}{5}-\frac{1}{5}\right)+...+\left(\frac{1}{13}-\frac{1}{13}\right)-\frac{1}{15}\)
\(=1-\frac{1}{15}=\frac{14}{15}\)
~ Hok tốt ~