\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{18.19}+\frac{2}{19.20}\)
\(=2.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{18.19}+\frac{1}{19.20}\right)\)
\(=2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{18}-\frac{1}{19}+\frac{1}{19}-\frac{1}{20}\right)\)
\(=2.\left(1-\frac{1}{20}\right)\)
\(=2.\frac{19}{20}=\frac{19}{10}\)
2/1x2 + 2/2x3 + 2/3x4 + .............+ 2/18x19 + 2/19x20
Bài 3:(nâng cao) tính nhanh:
\(\dfrac{2}{1x2}\) + \(\dfrac{2}{2x3}\) + \(\dfrac{2}{3x4}\) + ........ +\(\dfrac{2}{18x19}\) + \(\dfrac{2}{19x20}\)
tính tổng: S1= 1x2+2x3+3x4+.......+ 18x19+19x20+20x21
( giải ra cho mk nha
A,(2/1x2+2/2x3+2/3x4+...+2/18x19+2/19x20)-x=2/5
B, (1/21-1/30)x140+1,08:[0,2x(y-1)]=11
C, (x+1)+(x+5)+(x+9)+...+(x+29)=136
D, (y+2)+(y+4)+(y+6)+.....+(y+1996)=998000
tính tổng
1x2+2x3+3x4+...+19x20
tính nhanh
B=1x2+2x3+3x4+.....+18x19
bài 1 tính
a, 24 x 66 - 24 + 36 x 24
b, 29 x 173 - 29 x 72 -29
c, 1+5+10+15+...+55+60
d, 1x2+2x3+3x4 +...+18x19+19x20
bài 2 tìm x
a,106- ( x+ 7) =9
b, 2 x ( x+ 4) + 5 =65
c, (16x x -32) x 45=0
d, x+4 x x = 100 : 5
tính tổng của S1
S1= 1x2+2x3+3x4+....+19x20+20x21
Bài 1:Tính
M=1x2+2x3+3x4+.......+19x20
Ghi rõ lời giải ra cho mình nhé!
Thank you everyone!
