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TH1)
\(B=\dfrac{2}{11x15}+\dfrac{2}{15x19}+\dfrac{2}{19x23}+......+\dfrac{2}{51x55}\)
\(B=\dfrac{2}{11}-\dfrac{2}{15}+\dfrac{2}{15}-\dfrac{2}{19}+\dfrac{2}{19}-\dfrac{2}{23}+.....+\dfrac{2}{51}-\dfrac{2}{55}\)
\(B=\dfrac{2}{11}-\dfrac{2}{55}\)
\(B=\dfrac{8}{55}\)
TH2)
\(B=\dfrac{2}{11x15}-\dfrac{2}{15x19}-\dfrac{2}{19x23}-......-\dfrac{2}{51x55}\)
\(B=\dfrac{2}{11}+\dfrac{2}{15}-\dfrac{2}{15}+\dfrac{2}{19}-\dfrac{2}{19}+\dfrac{2}{23}-....-\dfrac{2}{51}+\dfrac{2}{55}\)
\(B=\dfrac{2}{11}+\dfrac{2}{55}\)
\(B=\dfrac{12}{55}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có:
A=2/4(4/11.15+4/15.19+4/19.23+.....+4/51.55)
A=2/4(1/11-1/15+1/15-1/19+1/19-1/23+....+1/51-1/55)
A=2/4(1/11-1/55)
A=2/4*4/55=8/220=2/55
B=-55/3/*8/3=-165/24=-55/8
suy ra A*B=2/55*(-55/8)=-1/4
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{2}{1.5}+\frac{3}{5.11}+\frac{4}{11.19}+\frac{5}{19.29}+\frac{6}{29.41}\)
\(A=\frac{1}{2}\left(\frac{4}{1.5}+\frac{6}{5.11}+\frac{8}{11.19}+\frac{10}{19.29}+\frac{12}{29.41}\right)\)
\(A=\frac{1}{2}\left(\frac{5-1}{1.5}+\frac{11-5}{5.11}+\frac{19-11}{11.19}+\frac{29-19}{19.29}+\frac{41-29}{29.41}\right)\)
\(A=\frac{1}{2}\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{11}+\frac{1}{11}-\frac{1}{19}+\frac{1}{19}-\frac{1}{29}+\frac{1}{29}-\frac{1}{41}\right)\)
\(A=\frac{1}{2}\left(1-\frac{1}{41}\right)=\frac{20}{41}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
M=1/3.7+1/7.11+1/11.15+...+1/43.47
M=1/3-1/7+1/7-1/11+1/11-1/15+...+1/43-1/47
M=1/3-1/47
CÒN LẠI TỰ TÍNH NHA BN
AI THẤY ĐÚNG THÌ ỦNG HỘ NHA
M=\(\frac{1}{3x7}+\frac{1}{7x11}+\frac{1}{11x15}+...+\frac{1}{43x47}\)
=>4M=\(\frac{4}{3x7}+\frac{4}{7x11}+...+\frac{4}{43x47}\)
=>4M=\(\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{43}-\frac{1}{47}\)
=>4M=\(\frac{1}{3}-\frac{1}{47}\)
=>4M=\(\frac{44}{141}\)
=>M=\(\frac{44}{141}:4\)
=>M=\(\frac{11}{141}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
bn viết đề mk chẳng hỉu gì
câu nào mk hỉu mk lm rùi đây (kq thui)
bài 1:
1, 55 2, 85 3, 1 4, 7/5 5, -17/144 6, 29/12
bài 2:
a,1+3+5+..+97+99 = 49.(99+1) = 49.100 =4900
b, 1+4+7+...+97+100 = 33.(100+1) = 33.101 =3333
c,d: chịu ^^ ><
\(C=\frac{2}{11.15}+\frac{2}{15.19}+\frac{2}{19.13}+...+\frac{2}{51.55}\)
\(2.C=\frac{4}{11.15}+\frac{4}{15.19}+\frac{4}{19.23}+...+\frac{4}{51.55}\)
\(2.C=\frac{1}{11}-\frac{1}{15}+\frac{1}{15}-\frac{1}{19}+\frac{1}{19}-\frac{1}{23}+...+\frac{1}{51}-\frac{1}{55}\)
\(2.C=\frac{1}{11}-\frac{1}{55}\)
\(2.C=\frac{5}{55}-\frac{1}{55}=\frac{4}{55}\)
\(C=\frac{4}{55}:2=\frac{4}{55}.\frac{1}{2}=\frac{2}{55}\)
Vậy \(C=\frac{2}{55}\)
đọc kĩ đi bạn ơi
có cả số 1 nữa