=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)……(1-1/10)(1+1/10)
=1/2 × 3/2×2/3 × 4/3×3/4 × 5/4×4/5×……×9/10 ×11/10
=1/2×11/10
=11/20