\(=\left(1-\frac{1}{2}\right)+\left(1-\frac{1}{6}\right)+\left(1-\frac{1}{12}\right)+......+\left(1-\frac{98}{90}\right)\)
\(=9-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+.....+\frac{1}{90}\right)\)
\(=9-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{9.10}\right)\)
\(=9-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+.....+\frac{1}{9}-\frac{1}{10}\right)\)
\(=9-\left(1-\frac{1}{10}\right)\)
\(=9-\frac{9}{10}\)
\(=\frac{81}{10}\)
(1-1/2)+(1-1/6)+(1-1/12)+(1-1/20)+(1-1/30)+(1-1/42)+(1-1/56)+(1-1/72)+(1-1/90)
=(1+1+1+1+1+1+1+1+1)-(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90)
=9-(1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7+1/7x8+1/8x9+1/9x10)
=9-(1-1/2+1/2+1/3+...............+1/9-1/10)
=9-(1-1/10)
=9-1/9
=80/9
B = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
B = (1 - 1/2) + (1 - 1/6) + (1 - 1/12) + ... + (1 - 1/90)
B = (1 - 1/1x2) + (1 - 1/2x3) + (1 - 1/3x4) + ... + (1 - 1/9x10)
B = (1 + 1 + 1 + ... + 1) - (1/1x2 + 1/2x3 + 1/3x4 + .. + 1/9x10)
B = 9 - (1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10)
B = 9 - (1 - 1/10)
B = 9 - 9/10
B = 81/10
Ủng hộ mk nha ♡_♡ ♧♧♧
