设二次函数ax^2+bx+c a不等于0
2f(x+2)-f(x-1)=2a(x+2)^2+2b(x+2)+2c-a(x-1)^2-b(x-1)-c
=2ax^2+8ax+8a+2bx+4b+2c-ax^2+2ax-a-bx+b-c
=ax^2+10ax+bx+7a+5b+c=ax^2+(10a+b)x+7a+5b+c=x^2+11x+13
a=1 a=1
10a+b=11 b=1
7a+5b+c=13 c=1
f(x)=x^2+x+1
设二次函数ax^2+bx+c a不等于0
2f(x+2)-f(x-1)=2a(x+2)^2+2b(x+2)+2c-a(x-1)^2-b(x-1)-c
=2ax^2+8ax+8a+2bx+4b+2c-ax^2+2ax-a-bx+b-c
=ax^2+10ax+bx+7a+5b+c=ax^2+(10a+b)x+7a+5b+c=x^2+11x+13
a=1 a=1
10a+b=11 b=1
7a+5b+c=13 c=1
f(x)=x^2+x+1