1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30+31+32+33+34+35+36
=(1+36)+(2+35)+(3+34)+.
=37+37+37+.(有36项,以2项为1组,共36/2=18组
=37*18
=666