\(\left(1-\frac{2}{42}\right)\left(1-\frac{2}{56}\right)...\left(1-\frac{2}{2652}\right)\)
= \(\frac{40}{42}.\frac{54}{56}.\frac{70}{72}...\frac{2650}{2652}\)
= \(\frac{5.8}{6.7}.\frac{6.9}{7.8}.\frac{7.10}{8.9}...\frac{50.53}{51.52}\)
= \(\frac{5.6.7...50}{6.7.8...51}.\frac{8.9.10...53}{7.8.9...52}\)
= \(\frac{5}{51}.\frac{53}{7}=\frac{265}{357}\)
Tính nhanh :
P =\(\frac{9,1+10,2+11,3+....+17,9+19}{\left(1-\frac{2}{42}\right)\left(1-\frac{2}{56}\right)\left(1-\frac{2}{72}\right)......\left(1-\frac{2}{2652}\right)}.\frac{265}{357}\)
tinh giùm cái A= 1/2+ 1/6 + 1/12 + 1/12 + 1/30 + 1/42 + 1/42 + 1/56 + 1/72
Tính nhanh :
A=1/30 + 1/42 + 1/56 + 1/72 + ... + 1/210
B=2/2.3 + 2/3.4 + 2/4.5 + ... + 2/99.100
B = 1/90 - 1/72 - 1/56 - 1/42 - 1/30 - 1/20 - 1/6 - 1/2
tính
A=1/12+1/20+1/30+1/42+1/56+1/72
B=2/1.4+2/4.7+2/7.10+2/10.13+2/13.16
1/90-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2 = ?
8/9 - 1/72 - 1/56 - 1/42 - 1/30 - 1/20 - 1/12 - 1/6 - 1/2
Tìm x biết: 2/42+2/56+2/72+....+2/x(x+1)=2/9
So sánh phân số A = 1/42+1/56+1/72+1/90+1/110+1/132 và B = (2/29-2/39+2/49)/(23/29-23/39+23/49)
