2/1+6/1+12/1+20/1+30/1+42/1
问题描述：
2/1+6/1+12/1+20/1+30/1+42/1
答
=1×2/1+2×3/1+3×4/1+4×5/1+5×6/1+6×7/1
=1/1-2/1+2/1-3/1+3/1-4/1+4/1-5/1+5/1-6/1+6/1-7/1
=1-7/1
=7/6