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
bạn nhé
tk nha@@@@@@@@@@@@@@@@@@@@
LOL
S = 21320
CHẮC LUÔN MK THI VIOLIMPIC RÙI
S = 1 x 2 + 2 x 3 + 3 x 4 + ....... + 38 x 39 + 39 x 40
S x 3 = 1 x 2 x 3 + 2 x 3 x 3 + 3 x 4 x 3 + ..... ++ 38 x 39 x 3 + 39 x 40 x 3
S x 3 = 1 x 2 x ( 3 - 0 ) + 2 x 3 x ( 4 - 1 ) + 3 x 4 x ( 5 - 2 ) +........ + 38 x 39 x ( 40 - 37 ) + 39 x 40 x ( 41 - 38 )
S x 3 = 1 x 2 x 3 - 1 x 2 x 0 + 2 x 3 x 4 - 2 x 3 x 1 + 3 x 4 x 5 - 2 x 3 x 4 + ..... + 38 x 39 x 40 - 38 x 39 x 37 + 39 x 40 x 41 - 39 x 40 x 38
S x 3 = ( 1 x 2 x 3 - 2 x 3 x 1 ) + ( 2 x 3 x 4 - 2 x 3 x 4 ) + ( 3 x 4 x 5 - 3 x 4 x 5 ) + ........ + ( 38 x 39 x 40 - 39 x 40 x 38 ) + 39 x 40 x 41
S x 3 = 39 x 40 x 41
S x 3 = 63960
S = 63960 : 3
S = 21320
Vậy : S = 1 x 2 + 2 x 3 + 3 x 4 + ..... + 38 x 39 + 39 x 40 = 21320
đáp án là:
S = 21230
chúc bạn hok tốt !!!
