1/n(n+2)=[1/n-1/(n+2)]÷2
7/1×3+7/3×5+7/5×7+……+7/99×101
=7×(1/1×3+1/3×5+1/5×7+.+1/99×101)
=7×(1-1/3+1/3-1/5+1/5-1/7+.+1/99-1/101)÷2
=7×(1-1/101)÷2
=350/101