a5 * a(2n-5) = 2^2n = a1 * a(2n-1) = a1a1q^(2n-2);
因为an > 0;所以:;a1q^(n-1) = an = 2^n;log2 an = n;原式 = 1 + 3 +.+ 2n-1;= n^2