1-3+5-7+9-11+13……-39+41

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1-3+5-7+9-11+13-...-39+41
=(1-3)+(5-7)+(9-11)+...+(37-39)+41
=(-2)X{[(39+1)/2]/2}+41
=-2X10+41
=21

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1-3+5-7+9-11+13…顷猛弯雀闷知李…-39+41
=1+(-3+5)+(-7+9)+(-11+13)+...+(-39+41)
=1+2+2+2+...+2
=1+2×10
=21

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