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A = 1 × 2 × 3 + 2 × 3 × 4 + .....+ 48 × 49 × 50
ta có 4 x A = 1 x 2 x 3 x 4 + 2 x 3 x 4 x (5 -1) + .....+ 48 × 49 × 50 x (51 - 47)
= 1 x 2 x 3 x 4 + 2 x 3 x 4 x 5 - 1 x 2 x 3 x 4 + ... + 48 x 49 x 50 x 51 - 47 x 48 x 49 x 50
= 48 x 49 x 50 x 51
suy ra A = (48 x 49 x 50 x 51) : 4
= 12 x 49 x 50 x 51
nhớ k cho mik nha rùi mik lm nốt cho
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{2}{2\cdot3\cdot4}+\frac{2}{3\cdot4\cdot5}+...+\frac{2}{47\cdot48\cdot49}+\frac{2}{48\cdot49\cdot50}\)
\(A=\frac{1}{2\cdot3}-\frac{1}{3\cdot4}+\frac{1}{3\cdot4}-\frac{1}{4\cdot5}+...+\frac{1}{47\cdot48}-\frac{1}{48\cdot49}+\frac{1}{48\cdot49}-\frac{1}{49\cdot50}\)
\(A=\frac{1}{2\cdot3}-\frac{1}{49\cdot50}\)
\(A=\frac{1}{6}-\frac{1}{2450}\)
\(A=\frac{611}{3675}\)
mong giúp đc bn.thk cho mk
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=1.2+2.3+...+48.49\)
\(\Rightarrow3A=1.2.3+2.3.3+...+48.49.3\)
\(=1.2.3+2.3.\left(4-1\right)+...+48.49.\left(50-47\right)\)
\(=1.2.3+2.3.4-1.2.3+....+48.49.50-47.48.49\)
\(=48.49.50\)
\(\Rightarrow A=\frac{48.49.50}{3}=39200\)
![](https://rs.olm.vn/images/avt/0.png?1311)
99 - 97 + 95 - 93 + 91 - 89 + ... + 7 - 5 + 3 -1
=(99 - 97) + (95 - 93) + (91 - 89) + ... + (7 - 5) + (3 -1) có 25 cặp số
=2+2+2+...+2+2 (có 25 số 2)
=2x25
=50
![](https://rs.olm.vn/images/avt/0.png?1311)
Day so tren co so cac so hang la
(50-1):1+1=50(so)
Tong cac so la:
(1+50)x50:2=1275
![](https://rs.olm.vn/images/avt/0.png?1311)
50 - 49 + 48 - 47 + 46 - 45 + ... + 4 - 3 + 2 - 1 ( có 50 số, 50 : 2 = 25)
= (50 - 49) + (48 - 47) + (46 - 45) + ... + (4 - 3) + (2 - 1) ( có 25 nhóm)
= -1 + (-1) + (-1) + .... + (-1) + (-1) ( có 25 số -1)
= -1 × 25
= -25
ta có: 50-49+48-47+46-45+....+4-3+2-1
=(50-49)+(48-47)+(46-45)+...+(4-3)+(2-1)
=1+1+1+1+....+1+1+1 (25 so)
=25
![](https://rs.olm.vn/images/avt/0.png?1311)
6.0828186e+70
1 x 2 x 3 x 4 x ... x 48 x 49 = 49! = gần = 6,08281864034267 x 10^ 62