当a是非零自然数时,它的倒数的2倍是2/a.1÷a ×2
=1/a ×2
=2/a原式=(5/6-2/5-1/3)×5/7
=(5/6-2/6 -2/5)×5/7
=(1/2-2/5)×5/7
=(5/10-4/10)×5/7
=1/10×5/7
=1/14