设an=kn+ba3=3k+b,a7=7k+b,a10=10k+b(a7)^2=(a3)(a10)49k^2+14kn+b^2=30k^2+13kb+b^219k^2+kb=019k+b=0b=-19k公比为a7/a3=(7k+b)/(3k+b)=(7k-19k)/(3k-19k)=12/16=3/4
