Question

Y=2x 3 21x 2 36x 10 find maximum and minimum value?

Answer 1

Answer:

Given, f(x)=2x

3

−21x

2

+36x−20

∴f

′

(x)=6x

2

−42x+36

When f(x) is a maximum or a minimum, f

′

(x)=0

Hence, 6x

2

−42x+36=0

x

2

−7x+6=0

x

2

−6x−x+6=0

x(x−6)−1(x−6)=0

(x−6)(x−1)=0

x=1,6

Again f

′′

(x)=12x−42

=6(2x−7)

Now, when x=1,f

′′

(x)=−30 ....[negative]

And when x=6,f

′′

(x)=30 ....[positive]

Hence, f(x) is maximum for x=1 and minimum for x=6.

The maximum and minimum values of f(x) are

f(1)=2(1)

3

−21(1)

2

+36(1)−20

=2−21+36−20=−3

f(6)=2(6)

2

−21(6)

2

+36(6)−20

=432−756+216−20=−128