Math Problem Statement

find the min and max values of the function y=(t-4t^2)^1/5 on the interval [-1,3] by comparing the critical points and endpoints

Solution

To find the minimum and maximum values of the function y=(t4t2)1/5y = (t - 4t^2)^{1/5} on the interval [1,3][-1, 3], we will follow these steps:

  1. Find the first derivative yy' of the function to locate the critical points.
  2. Evaluate the function at the critical points and the endpoints of the interval.
  3. Compare the function values to determine the minimum and maximum values.

Step 1: Find the first derivative

We will start by differentiating y=(t4t2)1/5y = (t - 4t^2)^{1/5} using the chain rule.

Let u=t4t2u = t - 4t^2, so y=u1/5y = u^{1/5}. The derivative of u1/5u^{1/5} with respect to tt is:

y=15u4/5dudty' = \frac{1}{5}u^{-4/5} \cdot \frac{du}{dt}

Now, compute dudt\frac{du}{dt}:

dudt=ddt(t4t2)=18t\frac{du}{dt} = \frac{d}{dt}(t - 4t^2) = 1 - 8t

Thus, the derivative becomes:

y=15(t4t2)4/5(18t)y' = \frac{1}{5}(t - 4t^2)^{-4/5} \cdot (1 - 8t)

Step 2: Find critical points

Critical points occur where y=0y' = 0 or where yy' does not exist (i.e., when the denominator is zero).

Set y=0y' = 0:

15(t4t2)4/5(18t)=0\frac{1}{5}(t - 4t^2)^{-4/5} \cdot (1 - 8t) = 0

Since the factor (t4t2)4/5(t - 4t^2)^{-4/5} is never zero (it only becomes undefined when t=0t = 0, but we will handle that separately), we need:

18t=01 - 8t = 0

Solving for tt:

8t=1t=188t = 1 \quad \Rightarrow \quad t = \frac{1}{8}

Thus, t=18t = \frac{1}{8} is a critical point.

Step 3: Evaluate the function at the critical point and endpoints

Now, evaluate y=(t4t2)1/5y = (t - 4t^2)^{1/5} at the endpoints of the interval [1,3][-1, 3] and at the critical point t=18t = \frac{1}{8}.

At t=1t = -1:

y(1)=(14(1)2)1/5=(14)1/5=(5)1/5y(-1) = (-1 - 4(-1)^2)^{1/5} = (-1 - 4)^{1/5} = (-5)^{1/5}

At t=3t = 3:

y(3)=(34(3)2)1/5=(336)1/5=(33)1/5y(3) = (3 - 4(3)^2)^{1/5} = (3 - 36)^{1/5} = (-33)^{1/5}

At t=18t = \frac{1}{8}:

y(18)=(184(18)2)1/5=(184164)1/5=(18116)1/5y\left(\frac{1}{8}\right) = \left(\frac{1}{8} - 4\left(\frac{1}{8}\right)^2\right)^{1/5} = \left(\frac{1}{8} - 4 \cdot \frac{1}{64}\right)^{1/5} = \left(\frac{1}{8} - \frac{1}{16}\right)^{1/5}

Simplifying:

y(18)=(216116)1/5=(116)1/5=(161)1/5=21/5y\left(\frac{1}{8}\right) = \left(\frac{2}{16} - \frac{1}{16}\right)^{1/5} = \left(\frac{1}{16}\right)^{1/5} = \left(16^{-1}\right)^{1/5} = 2^{-1/5}

Step 4: Compare values

Now we compare the values at the endpoints and the critical point.

  • y(1)=(5)1/5y(-1) = (-5)^{1/5}
  • y(3)=(33)1/5y(3) = (-33)^{1/5}
  • y(18)=21/5y\left(\frac{1}{8}\right) = 2^{-1/5}

The real values of negative numbers raised to fractional powers (such as (5)1/5(-5)^{1/5}) are real numbers, but they may be less straightforward to interpret directly without approximation.

Conclusion

By calculating these values or approximating them numerically, you can determine which one corresponds to the minimum and maximum on the interval.

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Math Problem Analysis

Mathematical Concepts

Calculus
Differentiation
Critical Points
Function Evaluation

Formulas

y = (t - 4t^2)^(1/5)
y' = 1/5 * (t - 4t^2)^(-4/5) * (1 - 8t)

Theorems

Chain Rule
Critical Point Theorem
Evaluating Functions at Critical Points

Suitable Grade Level

Grades 11-12