The first question is simple. Just calculate A 1= 1/2 and A2 = 1/6.

The second problem is traditional thinking:

Bring (Sn- 1) into the equation and get the square of Sn -(2+An)Sn+ 1 = 0; Find the serial number (represented by An)

Then use Sn-S(n- 1)= (the result of subtraction) =An, And you should be able to find an.

Mathematical induction:

An= 1/n(n+ 1) is guessed by A 1 and A2.

Suppose n= 1, k, k+ 1. This question is actually not difficult. Now the last day of the college entrance examination is often not the most difficult, so don't look at the last question of the college entrance examination.