21+22+23+···+28+29+30
=(21+30)x10÷2
=255
150-149+148-147+···+104-103+102-101
=(150-149)+(148-147)+···+(104-103)+(102-101)
=1+1+...+1+1
=25
21+22+23+···+28+29+30
=(21+30)x10÷2
=255
150-149+148-147+···+104-103+102-101
=(150-149)+(148-147)+···+(104-103)+(102-101)
=1+1+...+1+1
=25