1/1×4+1/4×7+1/7×10+1/10×13+1/13×16+1/16×19+1/19×22
=1/3(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13+1/13-1/16+1/16-1/19+1/19-1/22)
=1/3(1-1/22)
=1/3 *(21/22)
=7/22