y=ax^2-4x-13a有最小值
即a>0
y=ax^2-4x-13a
=a(x^2-4x/a+4/a^2)-4/a-13a
=a(x-2/a)^2-4/a-13a
-4/a-13a=-17
13a+4/a=17
13a^2+4=17a
13a^2-17+4=0
(13a-4)(a-1)=0
a=4/13或a=1