1十3十5十7十...37十39=400.用简便方法计算1+39=40,3+37=40,5+35=40,7+33=40,9+31=40,11+29=40,13+27=40,15+25=40,17+23=40,19+21=40,40*10=400.
1+3+..+39=(39+1)x10=400如有帮助请采纳，如对本题有疑问可追问，Good luck!
