穿针引线法
\( f(x) = 2ax^2+bx-3a+1 \) \( \forall{x} \in [-4,4] . f(x) \geq 0 \) \( 5a+b \) min
\( f(x) = (2x^2-3)a +xb+1 \)
\( \frac{2x^2-3}{1} = \frac{5}{1} \)
......
\(-\frac{1}{3}\)
