=2*1/2*2+2*2*1/2*3+2*3*1/2*4+.+2*24*1/2*25
=1/2+2/3+3/4+4/5+5/6+.24/25
=(1-1/2)+(1-1/3)+(1-1/4)+.(1-1/25)
=25-【(1+1/25)*25/2]
=12
=2*1/2*2+2*2*1/2*3+2*3*1/2*4+.+2*24*1/2*25
=1/2+2/3+3/4+4/5+5/6+.24/25
=(1-1/2)+(1-1/3)+(1-1/4)+.(1-1/25)
=25-【(1+1/25)*25/2]
=12