Question

1-2+3-4+5-6+7-8加到21等于多少?

Answer 1

(1-2)+(3-4)+(5-6)+(7-8)+(9-10)+(11-12)+(13-14)+(15-16)+(17-18)+(19-20)+21=-1×10+21=-20+21=1

Answer 2

1-2+3-4+5-6+7-8...加到21
1+3+5+7+9...+21=121

-2-4-6-8..-20=-98
121+（-98）=23

Answer 3

1-2+3-4+5-6+7-8+...+19-20+21=-1-1-...-1(10个－1)+21=-10+21=11

追问

没看懂，具体怎么做的

追答

1-2+3-4+5-6+7-8+...+19-20+21
=(1-2)+(3-4)+(5-6)+(7-8)+...+(19-20)+21
=-1-1-...-1(10个－1)+21
=-10+21
=11

Answer 4

11