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More from Application of Derivatives

Evaluate Fourth Degree Polynomial with Given Extrema and Limit

Let p(x)p(x) be a polynomial of degree 4 having extremum at x=1,2x = 1, 2 and
lim⁡x→0(1+p(x)x2)=2.\lim_{x \to 0} \left( 1 + \frac{p(x)}{x^2} \right) = 2.
Then the value of p(2)p(2) is

Official Numerical Answer0

Step-by-Step Solution

To find the value of p(2)p(2), we begin by analyzing the given limit:
lim⁡x→0(1+p(x)x2)=2\lim_{x \to 0} \left( 1 + \frac{p(x)}{x^2} \right) = 2

Subtracting 11 from both sides gives:
lim⁡x→0p(x)x2=1\lim_{x \to 0} \frac{p(x)}{x^2} = 1

Since p(x)p(x) is a polynomial of degree 4, we can express it in standard form as:
p(x)=ax4+bx3+cx2+dx+ep(x) = ax^4 + bx^3 + cx^2 + dx + e

For the limit lim⁡x→0p(x)x2\lim_{x \to 0} \frac{p(x)}{x^2} to exist and be finite, the terms of degree less than 2 must be zero:
e=0andd=0e = 0 \quad \text{and} \quad d = 0

Furthermore, the coefficient of x2x^2 is given by the value of the limit:
c=1c = 1

Thus, the polynomial takes the form:
p(x)=ax4+bx3+x2p(x) = ax^4 + bx^3 + x^2

Next, we calculate the first derivative of p(x)p(x):
p′(x)=4ax3+3bx2+2xp'(x) = 4ax^3 + 3bx^2 + 2x

Since p(x)p(x) has extrema at x=1x = 1 and x=2x = 2, we have p′(1)=0p'(1) = 0 and p′(2)=0p'(2) = 0:

  1. For x=1x = 1:
    p′(1)=4a(1)3+3b(1)2+2(1)=0  ⟹  4a+3b+2=0  ⟹  4a+3b=−2p'(1) = 4a(1)^3 + 3b(1)^2 + 2(1) = 0 \implies 4a + 3b + 2 = 0 \implies 4a + 3b = -2

  2. For x=2x = 2:
    p′(2)=4a(2)3+3b(2)2+2(2)=0  ⟹  32a+12b+4=0p'(2) = 4a(2)^3 + 3b(2)^2 + 2(2) = 0 \implies 32a + 12b + 4 = 0
    Dividing this equation by 4 gives:
    8a+3b=−18a + 3b = -1

Now, subtracting the first equation from the second equation:
(8a+3b)−(4a+3b)=−1−(−2)(8a + 3b) - (4a + 3b) = -1 - (-2)
4a=1  ⟹  a=144a = 1 \implies a = \frac{1}{4}

Substituting a=14a = \frac{1}{4} back into 4a+3b=−24a + 3b = -2:
4(14)+3b=−2  ⟹  1+3b=−2  ⟹  3b=−3  ⟹  b=−14\left(\frac{1}{4}\right) + 3b = -2 \implies 1 + 3b = -2 \implies 3b = -3 \implies b = -1

Therefore, the polynomial is:
p(x)=14x4−x3+x2p(x) = \frac{1}{4}x^4 - x^3 + x^2

Finally, we evaluate p(2)p(2):
p(2)=14(2)4−(2)3+(2)2=14(16)−8+4=4−8+4=0p(2) = \frac{1}{4}(2)^4 - (2)^3 + (2)^2 = \frac{1}{4}(16) - 8 + 4 = 4 - 8 + 4 = 0