The smallest value of the polynomial x3 – 18x2 + 96x in [0, 9] is
Let f(x)= x3 – 18x2 + 96x
Applying the first derivative we get
Applying the sum rule of differentiation, so we get
Applying the derivative,
⇒ f' (x)=3x2-36x+96
Putting f’(x)=0,we get critical points
3x2-36x+96=0
⇒ 3(x2-12x+32)=0
⇒ x2-12x+32=0
Splitting the middle term, we get
⇒ x2-8x-4x+32=0
⇒ x(x-8)-4(x-8)=0
⇒ (x-8)(x-4)=0
⇒ x-8=0 or x-4=0
⇒ x=8 or x=4
⇒ x∈[0,9]
Now we will find the value of f(x) at x=0, 4, 8, 9
f(x)= x3 – 18x2 + 96x
f(0)= 03 – 18(0)2 + 96(0)=0
f(4)= 43 – 18(4)2 + 96(4)=64-288+384=160
f(8)= 83 – 18(8)2 + 96(8)=512-1152+768=128
f(9)= 93 – 18(9)2 + 96(9)=729-1458+864=135
Hence we find that the absolute minimum value of f(x) in [0,9] is 0 at x=0.
So the correct option is option B.