a 1q^(n+ 1)+a 1q？ -2a 1q^(n- 1)=0，

So q? +q-2=0, and the solution is q=-2 (excluding 1).

So a 1=a3/q? = 1,

Therefore S7 = (1-q 7)/( 1-q).

= 129/3=43 。