=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/77)(1+1/77) 先分解因式,平方差公式=(1-1/2)(1-1/3)(1-1/4)……(1-1/77)(1+1/2)(1+1/3)(1+1/4)……(1+1/77) 减的写在前面=1/2×2/3×3/4……76/77 ×3/2×4/3×5/4……×78/7...