1/4×5+1/5×6+1/6×7+……1/39×40
=(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+.+(1/39-1/40)
=1/4-1/5+1/5-1/6+1/6-1/7+.+1/39-1/40
=1/4-1/40
=9/40