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1920+(−1820)+1720+(−1620)+ ... +(−420)+320+(−220)+120=

pls

Sep 19, 2019
edited by Guest  Sep 19, 2019

#1
+24376
+2

1920+(-1820)+1720+(-1620)+ ... +(-420)+320+(-220)+120=

$$\small{ \begin{array}{|rcll|} \hline && \underbrace{1920-1820}_{=100} +\underbrace{1720-1620}_{=100} +\underbrace{1520-1420}_{=100} +\underbrace{1320-1220}_{=100} +\underbrace{1120-1020}_{=100} +\underbrace{920-820}_{=100} +\underbrace{720-620}_{=100} +\underbrace{520-420}_{=100} +\underbrace{320-220}_{=100} +120 \\ &=& 9\cdot 100 + 120 \\ &=& 900 + 120 \\ &=& \mathbf{1020} \\ \hline \end{array} }$$

Sep 19, 2019

#1
+24376
+2
$$\small{ \begin{array}{|rcll|} \hline && \underbrace{1920-1820}_{=100} +\underbrace{1720-1620}_{=100} +\underbrace{1520-1420}_{=100} +\underbrace{1320-1220}_{=100} +\underbrace{1120-1020}_{=100} +\underbrace{920-820}_{=100} +\underbrace{720-620}_{=100} +\underbrace{520-420}_{=100} +\underbrace{320-220}_{=100} +120 \\ &=& 9\cdot 100 + 120 \\ &=& 900 + 120 \\ &=& \mathbf{1020} \\ \hline \end{array} }$$