已知f(x)=ax2+bx,且1<=f(-1)<=2,2<=f(1)<=4,求f(-2)的取值范围.
带入可的1<=a-b<=2 （１）2<=a+b<=4 （２）求4a-2b的范围把(1)*3+(2)就可以得到了5<=f(-2)<=10
