數學問題!

1) 2+4+6+......+74+76+78
=2(1+2+3+4+5+6......+37+38+39)
=2[(1+39)(39/2)]
=2(40)(19.5)
=2(780)
=1560

2) 15+17+19+.....+33+35+37
=(15+37)[(37-15)/2+1]/2
=52(22/2+1)/2
=52(12)/2
=52(6)
=312

1.1560
2.312
3.774

I hope can help you.

1) 2+4+6+......+74+76+78
=(2+78)×[(78-2)÷2+1]
=80×39
=3120
2) 15+17+19+.....+33+35+37=?
=(15+37)×[(37-15)÷2+1]
=52×12
=624
3) 26+27+28+.....+46+47+48
=(26+48)×(48-26+1)
=74×23
=1702

1 1400
2 312
3 851

解 :
1.1560
2.312
3.774

1.(2+78)[(78-2)/2+1]/2

=80*39/2

=40*39

=1560

2.(15+37)[(37-15)/2+1]/2

=52*7/2

=26*7

=182

3.(26+48)[(48-26)/1+1]/2

=74*23/2

=37*23

=774

1.1560
2.312
3.774