1) \(1+\frac{1}{3}+1\frac{91}{27}+\frac{1}{81}\)
\(=1+\frac{1}{3}+4+\frac{10}{27}+\frac{1}{81}\)
\(=\left(1+4\right)+\left(\frac{1}{3}+\frac{10}{27}+\frac{1}{81}\right)\)
\(=5+\frac{58}{81}=5\frac{58}{81}\)
2) \(\frac{1}{78}+\frac{1}{91}+\frac{1}{105}+\frac{1}{120}+\frac{1}{136}\)
\(=\frac{1}{6\times13}+\frac{1}{13\times7}+\frac{1}{7\times15}+\frac{1}{15\times8}+\frac{1}{8\times17}\)
\(=\frac{1}{6}-\frac{1}{13}+\frac{1}{13}-\frac{1}{7}+\frac{1}{7}-\frac{1}{15}+\frac{1}{15}-\frac{1}{8}+\frac{1}{8}-\frac{1}{17}\)
(gạch bỏ các số giống nhau)
\(=\frac{1}{6}-\frac{1}{17}=\frac{11}{102}\)
3) \(\frac{1}{6}+\frac{1}{30}+\frac{1}{70}+\frac{1}{126}\)
\(=\frac{1}{2\times3}+\frac{1}{3\times10}+\frac{1}{10\times7}+\frac{1}{7\times18}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{10}+\frac{1}{10}-\frac{1}{7}+\frac{1}{7}-\frac{1}{18}\)
\(=\frac{1}{2}-\frac{1}{18}=\frac{4}{9}\)
chúc bn học tốt!!
công chúa ôri ơi mk viết nhầm đề bài 1
đề bài là 1+1/3+1/9+1/27+1/81
bạn trả lời giúp mk với mk cần gấp lắm
cảm ơn
a)\(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\)
\(=1+\frac{1}{1\times3}+\frac{1}{3\times3}+\frac{1}{3\times9}+\frac{1}{9\times9}\)
\(=1+1-\frac{1}{3}+\frac{1}{3}-\frac{1}{3}+\frac{1}{3}-\frac{1}{9}+\frac{1}{9}-\frac{1}{9}\)
\(=2-\frac{1}{9}=1\frac{8}{9}\)
BN VIẾT NHẦM CÂU 1 , THẢO NÀO MK THẤY RA SỐ TO QUÁ!