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S = 1 x 2 +2x3 + 3x4+... + 38x39+39x40
3S= 1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3
3S=1.2.(3-0)+2.3(4-1)+...+38.39(40-37)
3S=0.1.2-1.2.3+1.2.3-2.3.4+...+37.39.39-38.39.40
3S=38.39.40
S=19760
S = 1 x 2 +2x3 + 3x4+... + 38x39+39x40
3S= 1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3
3S=1.2.﴾3‐0﴿+2.3﴾4‐1﴿+...+38.39﴾40‐37﴿
3S=0.1.2‐1.2.3+1.2.3‐2.3.4+...+37.39.39‐38.39.40
3S=38.39.40
S=19760
S= 1.2 + 2.3 + ... + 39.40
=> 3S = 3.(1.2 + 2.3 + ... + 39.40)
=> 3S = 1.2.(3-0) + 2.3.(4-1) +....+39.40.(41-38)
=> 3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + ... + 39.40.41 - 38.39.40
=> 3S = 39.40.41
=> S = \(\frac{39.40.41}{3}\)
=> S=21320
S=1.2+2.3+3.4+.....+38.39+39.40
=>3S=1.2.3+2.3.3+3.3.4+...+3.38.39+39.40.3
=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+38.39.40-37.38.39+39.40.41-38.39.40
=39.40.41
=>S=13.40.41
S= 1.2+2.3+3.4+.....+38.39+39.40
3.S= 3.(1.2+2.3+3.4+.....+38.39+39.40)
3S = 1.2.3+2.3.3+3.3.4+......+3.38.39+3.39.40
3S = 1.2.3+2.3.(4-1)+3.4.(5-2)+.....+38.39.(40-37)+39.40.(41-38)
3S = 1.2.3+2.3.4-2.3.1+3.4.5-3.4.2+......+38.39.40-38.39.37+39.40.41-39.40.38
3S = (1.2.3-2.3.1)+(2.3.4-3.4.2)+.....+(38.39.40-39.40.38)+39.40.41
3S = 39.40.41
S = (39.40.41) :3
S = (39:3).40.41
S = 13.40.41=21320
* Chúc bạn học tốt
\(S=\) 1x2 + 2x3 + 3x4 +...+ 38x39 + 39x40
\(\Rightarrow3S\) = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + .... + 38x39x(40-37) + 39x40x(41-38)
\(\Rightarrow3S=\) 1x2x3 - 1x2x3 + 2x3x4 - 2x3x4 +3x4x5 +.... + 38x39x40 - 37x38x39+ 39x40x41-38x39x40
\(\Rightarrow3S=\)39x40x41 = 63 960
\(\Rightarrow S\) = 63960 : 3 = 21320
S = 1.2+2.3+3.4+....+38.39+39.40
S.3 = ( 1.2+2.3+3.4+...+38.39+39.40 ) . 3
S.3 = 1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...........+38.39.(40-37)+39.40.(41-38)
S.3 = 1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...........+38.39.40-37.38.39+39.40.41-38.39.40
S = \(\frac{39.40.41}{3}\)
S = 13.40.41
S = 21320
\(A=\frac{1.98+2.97+3.96+...+98.1}{1.2+2.3+3.4+...+98.99}=\frac{1.\left(100-2\right)+2\left(100-3\right)+3\left(100-4\right)+...+98\left(100-99\right)}{1.2+2.3+3.4+...+98.99}\)
\(A=\frac{1.100-1.2+2.100-2.3+3.100-3.4+...+98.100-98.99}{1.2+2.3+3.4+...+98.99}\)
\(A=\frac{\left(1.100+2.100+3.100+...+98.100\right)-\left(1.2+2.3+3.4+...+98.99\right)}{1.2+2.3+3.4+...+98.99}\)
\(A=\frac{100\left(1+2+3+...+98\right)}{1.2+2.3+3.4+...+98.99}-1\)
Ta có: 1+2+3+...+98=98.99:2=4851
Đặt B=1.2+2.3+3.4+...+98.99 => 3B=1.2.3+2.3.3+3.4.3+...+98.99.3 = 1.2.3+2.3.(4-1)+3.4(5-2)+...+98.99(100-97)
=> 3B=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99 = 98.99.100
=> B=33.98.100. Thay vào A được:
\(A=\frac{100.4851}{33.98.100}-1=\frac{3}{2}-1=\frac{1}{2}\)
S=1.2+2.3+...+39.40
3S=1.2.3+2.3.3+...+39.40.3
3S=1.2.(3-0)+2.3.(4-1)+...+39.40.(41-38)
3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40
3S=39.40.41
S=\(\frac{39.40.41}{3}\)
=>S=21320
\(S=1.2+2.3+3.4+...+39.40\)
\(\Rightarrow3S=3\left(1.2+2.3+3.4+...+39.40\right)\)
\(\Rightarrow3S=3.1.2+3.2.3+3.3.4+...+3.39.40\)
\(\Rightarrow3S=\left(3-0\right).1.2+\left(4-1\right).2.3+\left(5-2\right).3.4+...+\left(41-38\right).39.40\)
\(\Rightarrow3S=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+39.40.41-38.39.40\)
\(\Rightarrow3S=39.40.41\)
\(\Rightarrow S=\frac{39.40.41}{3}=21320\)
21320 đúng ko
gợi ý có 5 chữ số