1-3+2-6+3-9+4-12+.+48-144+49-147+50
=1+2+3+……+50-3-6-9-12-……-144-147-150+150
=(1+2+3+……+50)-(3+6+9+12+……147+150)+150
=(1+2+3+……+50)-3(1+2+3+……+50)+150
=-2(1+2+3+……+50)+150
=-2x25x51+150
=-2400
1-3+2-6+3-9+4-12+.+48-144+49-147+50
=1+2+3+……+50-3-6-9-12-……-144-147-150+150
=(1+2+3+……+50)-(3+6+9+12+……147+150)+150
=(1+2+3+……+50)-3(1+2+3+……+50)+150
=-2(1+2+3+……+50)+150
=-2x25x51+150
=-2400