5/(2*1)+5/(2*3)+5/(3*4)……5/(49*50)
=5*(1/1*2+1/2*3+1/3*4+.1/49*50)
=5*(1-1/2+1/2-1/3+1/3-1/4+1/4-.-1/49+1/49-1/50)
=5*(1-1/50)
=5*49/50
=49/10
=4.9