(-2)/1*3+(-2)/3*5+(-2)/5*7+...+(-2)/29*31
=[(-2)-(-2)/3+(-2)/3-(-2)/5+(-2)/5-(-2)/7+...+(-2)/29-(-2)/31]
=[(-2)-(-2)/31]=-60/31.
6/8*10+6/10*12+6/168+...+6/2400
=2*(2/8*10+2/10*12+2/12*14+...+2/48*50)
=2*(2/8-2/10+2/10-2/12+2/12-2/14+...+2/48-2/50)
=2*(2/8-2/50)=2*(1/4-1/25)=2*21/100=21/50.