Câu hỏi của Hỏa Hỏa

Question

Tính:

Lightning Farron · Accepted Answer

$S=1\cdot2+2\cdot3+3\cdot4+...+39\cdot40$

$3S=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+3\cdot4\cdot\left(5-2\right)+...+39\cdot40\cdot\left(41-38\right)$

$3S=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+...+39\cdot40\cdot41-38\cdot39\cdot40$

$3S=39\cdot40\cdot41$$\Rightarrow S=\dfrac{39\cdot40\cdot41}{3}=21320$Câu trả lời: $S=1\cdot2+2\cdot3+3\cdot4+...+39\cdot40$

$3S=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+3\cdot4\cdot\left(5-2\right)+...+39\cdot40\cdot\left(41-38\right)$

$3S=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+...+39\cdot40\cdot41-38\cdot39\cdot40$

$3S=39\cdot40\cdot41$$\Rightarrow S=\dfrac{39\cdot40\cdot41}{3}=21320$

Eren Jeager · Answer

Ta có : $S=1.2+2.3+3.4+...+38.39+39.40$

$3S=1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3$

$3S=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+39.40.\left(41-38\right)$          $3S=39.40.41$

$S=\dfrac{39.40.41}{3}$$=21320$Câu trả lời: Ta có : $S=1.2+2.3+3.4+...+38.39+39.40$

$3S=1.2.3+2.3.3+3.4.3+...+38.39.3+39.40.3$

$3S=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+39.40.\left(41-38\right)$          $3S=39.40.41$

$S=\dfrac{39.40.41}{3}$$=21320$

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