a2*a3=2a1即a1q*a1q^2=2a1 即有a1q^3=2a4与2a7的等差中项为5/4a1q^3+2a1q^6=5/4*2联立求解：a1= 16 q=1/2S5=1
a1²q^3=2a1a1q^3=2a4 =2a4+2a7=2*5/4=5/22+2a7=5/2a7=1/4a7/a3=1/8=q^3q=1/2a1=16S5=16+8+4+2+1=31
