Question

计算（简算） 1/4×6+1/6×8+1/8×10+1/10×12+1/12×14

Answer 1

1/4×6+1/6×8+1/8×10+1/10×12+1/12×14
=1/2×(1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12+1/12-1/14)
=1/2×(1/4-1/14)
=1/2×(7/28-2/28)
=1/2×5/28
=5/56

追问

1/2哪来的？