3²/2×4+5²/4×6+7²/6×8+9²/8×10+11²/10×12
=5+1/2×4+1/4×6+1/6×8+1/8×10+1/10×12
=5+（1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12）÷2
=5+（1/2-1/12）÷2
=5+5/24
=5又5/24