Accedmychevron_right11thchevron_rightmathchevron_rightFunctions And Graphschevron_rightExercise 2.1

Solution

Given:

f(x)=x3ax2+bx+1f(x)=x^3-ax^2+bx+1

Use f(2)=3f(2)=-3

f(2)=23a(22)+b(2)+1=384a+2b+1=394a+2b=34a+2b=122a+b=6(1)\begin{aligned} f(2)&=2^3-a(2^2)+b(2)+1=-3\\ 8-4a+2b+1&=-3\\ 9-4a+2b&=-3\\ -4a+2b&=-12\\ -2a+b&=-6\quad (1) \end{aligned}

Use f(1)=0f(-1)=0

f(1)=(1)3a(1)2+b(1)+1=01ab+1=0ab=0a+b=0(2)\begin{aligned} f(-1)&=(-1)^3-a(-1)^2+b(-1)+1=0\\ -1-a-b+1&=0\\ -a-b&=0\\ a+b&=0\quad (2) \end{aligned}

From (2): b=ab=-a.

Substitute into (1):

2a+(a)=63a=6a=2-2a+(-a)=-6\Rightarrow -3a=-6\Rightarrow a=2

Then b=2b=-2.

a=2,b=2\boxed{a=2,\quad b=-2}