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a, 1+2-3+4-5+...+88-89+90

b, 1+2-3-4-5+...+89+90

Viên Tiến Duy · Accepted Answer

1+2-3+4-5+...+88-89 + 90

= 1 + ( 2 + 4 +...+90) - ( 3+5++...+89)

= 1 + $\dfrac{\left(90+2\right)\times45}{2}$-  $\dfrac{\left(89+3\right)\times44}{2}$

= 1 + $92\times\left(\dfrac{45}{2}-\dfrac{44}{2}\right)$

= 1 + $\dfrac{92}{2}$

= 1 + 46

= 47Câu trả lời: 1+2-3+4-5+...+88-89 + 90

= 1 + ( 2 + 4 +...+90) - ( 3+5++...+89)

= 1 + $\dfrac{\left(90+2\right)\times45}{2}$-  $\dfrac{\left(89+3\right)\times44}{2}$

= 1 + $92\times\left(\dfrac{45}{2}-\dfrac{44}{2}\right)$

= 1 + $\dfrac{92}{2}$

= 1 + 46

= 47