1 / 12 + 1 / 20 + 1 / 30 + 1 / 42 + 1 / 56 + 1 / 72 + 1 / 90 + 1 / 110 + 1 / 132 =
(2 310 * 1) / (2 310 * 12) + (1 386 * 1) / (1 386 * 20) + (924 * 1) / (924 * 30) + (660 * 1) / (660 * 42) + (495 * 1) / (495 * 56) + (385 * 1) / (385 * 72) + (308 * 1) / (308 * 90) + (252 * 1) / (252 * 110) + (210 * 1) / (210 * 132) =
2 310 / 27 720 + 1 386 / 27 720 + 924 / 27 720 + 660 / 27 720 + 495 / 27 720 + 385 / 27 720 + 308 / 27 720 + 252 / 27 720 + 210 / 27 720 =
( 2 310 + 1 386 + 924 + 660 + 495 + 385 + 308 + 252 + 210 ) / 27 720 = 6 930 / 27 720
Đề thiếu chắc mk làm máy bài này rồi !
A=\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{11}{110}\)
\(A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{11}{110}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}+\frac{11}{110}\)
\(A=1-\frac{1}{7}+\frac{11}{110}\)
\(A=\frac{6}{7}+\frac{11}{110}\)
\(A=\frac{67}{70}\)
\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{110}\)= \(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{110}\)
= \(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{110}\)
= \(\frac{1}{2}-\frac{1}{7}+\frac{1}{110}\)= \(\frac{141}{385}\)