1*5分之1+5*9分之1+9*13分之1+.+2

1个回答

1*5分之1+5*9分之1+9*13分之1+.+2005*2009分之1
=1/4[1-1/5]+1/4[1/5-1/9]+...+1/4[1/2005-1/2009]
=1/4[1-1/5+1/5-1/9+...+1/2005-1/2009]
=1/4[1-1/2009]
=502/2009