(a) Since (x+2) and (2x+3) are factors of f(x), we can use the factor theorem. This means that f(−2)=0 and f(−23)=0. f(−2)=6(−2)3+p(−2)2+4(−2)−q=0 −48+4p−8−q=0 4p−q=56 (Equation 1) f(−23)=6(−23)3+p(−23)2+4(−23)−q=0 6(−827)+p(49)−6−q=0 −481+49p−6−q=0 Multiply by 4 to get rid of fractions:
−81+9p−24−4q=0 9p−4q=105 (Equation 2) Now we solve the system of equations:
4p−q=56 9p−4q=105 From Equation 1, q=4p−56. Substitute this into Equation 2: 9p−4(4p−56)=105 9p−16p+224=105 −7p=−119 Now substitute p=17 into q=4p−56: q=4(17)−56=68−56=12 So, p=17 and q=12. (b) Now we know f(x)=6x3+17x2+4x−12. We know that (x+2) and (2x+3) are factors. Since f(x) is a cubic polynomial, it has three roots. We already know two roots are x=−2 and x=−23. We want to find the third root. Since (x+2) and (2x+3) are factors, (x+2)(2x+3)=2x2+7x+6 is a factor of f(x). We can perform polynomial division to find the other factor.
(6x3+17x2+4x−12)/(2x2+7x+6)=3x−2 So f(x)=(x+2)(2x+3)(3x−2). Thus, the roots are x=−2, x=−23, and x=32.