用平方差公式：1-1/2²=1²-(1/2)²=(1-1/2)×(1+1/2)=1/2×3/21-1/3²=1²-(1/3)²=(1-1/3)×(1+1/3)=2/3×4/31-1/4²=1²-(1/4)²=(1-1/4)×(1+1/4)=3/4×5/41-1/5²=1&s...