According to a9+a 10=a, we can get (1+q) * q9 = a. Similarly, (1+q) * a19 = (1+q) * a9. . .

2、

First, the square of a6*a2=a4 is because a6=a 1*q is the fifth power, a2=a 1*q, a4=a 1*q is the third power; Then A6 * A2 = the square of A65438 +0 times the sixth power of Q = the square of A4. This is the basic property of geometric series, which must be found in books.

Similarly, the square of a6 = A4 * A8a6 * a10 = the square of A8, so the required formula is your answer.

3、

Because an =1/n (n+1) =1/n-1(n+1), then Sn = A 1+A2+A3++. 4 ++ 1/n- 1/(n+ 1)= 1- 1/( 1+n)= n/(n+65438+)