1/2(f(x1)+f(x2))-f((x1+x2)/2)=1/2(a^x1+a^x2)-a^((x1+x2)/2)
=1/2(a^(x1/2)-a^(x2/2))^2>=0
所以1/2(f(x1)+f(x2))>f((x1+x2)/2),当x1=x2时相等
1/2(f(x1)+f(x2))-f((x1+x2)/2)=1/2(a^x1+a^x2)-a^((x1+x2)/2)
=1/2(a^(x1/2)-a^(x2/2))^2>=0
所以1/2(f(x1)+f(x2))>f((x1+x2)/2),当x1=x2时相等