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