21+22+23+24+25+26+27+28+29+30+31+32+33+34+35+36+37+38+39+40=?
问题描述:
21+22+23+24+25+26+27+28+29+30+31+32+33+34+35+36+37+38+39+40=?
答
=(21+40)+(22+39)+……+(30+31)
=61+61+……+61
=61×10
=610