3-1+7-4+11-7+15-10+..-70+99
=(3-1)+(7-4)+(11-7)+(15-10)+..(95-70)+99
=2+3+4+5+……+24+25+99
=(2+25)×24÷2+99
=27×12+99
=324+99
=423
3-1+7-4+11-7+15-10+..-70+99
=(3-1)+(7-4)+(11-7)+(15-10)+..(95-70)+99
=2+3+4+5+……+24+25+99
=(2+25)×24÷2+99
=27×12+99
=324+99
=423