[toán 8] Tính

H

huy14112



$-1^2+2^2-3^2+4^2+....-19^2+20^2$

$=(2-1)(2+1)+(4-3)(4+3)+....................+(20-19)(20+19)$

$=1.3+1.7+1.11+..........+1.39$

$=3+7+11+........+39$

$=[(39-3):4+1]:2.(39+3)$

$=210$
 
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