Gọi tổng trên là A.Ta có:
A=5/2x4+5/4x6+...+5/98x100
1/2A=5/2-5/4+5/4-5/6+...+5/98-5/100
1/2A=5/2-5/100
1/2A=250/100-5/100
1/2A=49/20
A=49/20:1/2
A=49/20.2
A=98/20=49/10
2-1+4-3+6-5+.............+1000-999
1/2*4+1/4*6+.............+1/98*1000
5/2*4+5/4*6+...........+5/98*100
tính nhanh (2/3+3/4+5/6+...+99/100).(1/2+2/3+3/4+...+98/99)-(1/2+1/3+...+99/100).(2/3+2/4+...+98/99)
1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+...+1/98*99+1/99*100
1*2*3*4*5*6.....*98*99*100
1+2+3+4+5+6+7+8...+98+99+100=
A= 2-3+4-5+6-7+8-9...+98-99+100
A=2-3+4-5+6-7+8-9+...+98-99+100
a=2-3+4-5+6-7+8-9+.......+98-99+100
a = 2 - 3 + 4 - 5 + 6 - 7 + 8 - 9 ... + 98 - 99 + 100=?
