A = 3/10x11 - 3/11x12 - .........- 3/99x100
A = 3 x ( 1/10x11 - 1/11x12 - ..... - 1/99x100 )
A = 3 x ( 11 - 10 / 10 x 11 - 12 - 11 / 11 x 12 - ...... - 100 - 99 /99 x 100 )
A = 3 x ( 11/10x11 - 10/10x11 + 12/11x12 - 11/11x12 + ......+ 100/99x100 - 99/99x100)
A = 3 x ( 1/10 - 1/11 + 1/11 - 1/12 + ......+ 1/99 - 1/100 )
A = 3 x ( 1/10 - 1/100)
A = 3 x 9/100
A = 27/100
TÍNH NHANH:
A=1/10X11+1/11X12+.....+1/99X100
\(\frac{3}{1x2}+\frac{3}{2x3}+\frac{3}{3x4}+...+\frac{3}{99x100}\)
Thực hiện phép tính
A=10x11+11x12+12x13+......+29x30
A = \(\dfrac{7}{10x11}\) + \(\dfrac{7}{11x12}\) + \(\dfrac{7}{12x13}\) +... + \(\dfrac{7}{69x70}\)
Tính tổng
A=7/10x11+7/11x12+7/12x13+...+7/69x70
Tính các tổng sau: A=7/10x11 + 7/11x12 + 7/12x13 +...+ 7/69x70
b, B = 1\(\frac{1}{1x2}+\frac{1}{2x3}+......+\frac{1}{99x100}\)
c, C = \(\frac{1}{1x2}+\frac{1}{2x3}+......+\frac{1}{n\left(n+1\right)}\)
d, D = 1 + 2 + 3 + ......+ n
C=7/10x11+7/11x12+7/12x13+.................+7/69x70 =?
\(\frac{1}{1x2}+\frac{1}{2x3}+.....+\frac{1}{99x100}\)
