1. 2.2 Power Functions With Modeling
Sketch power functions in the form f(x) = kxn (where k and n are rational numbers).
Use power functions to model real-life data and use these models to make predictions

2. What is a rational number?
Power Functions with Modeling
What is a rational number?
Any number which can be expressed as a fraction is a rational number.
You may have learned this definition as any number which can be expressed as p/q where q  0.

3. Power Functions with Modeling
Any function that can be written in the form f(x) = kxn, where k and n are nonzero constants, is a power function.
The constant n is the power, and k is the constant of variation, or constant of proportion.
Note: We say f(x) varies as the nth power of x, of f(x) is proportional to the nth power of x.

4. Common Formulas which are power functions
Power Functions with Modeling
Common Formulas which are power functions
Name
Formula
Power
Constant of Variation
Circumference
C = 2pr 1 2p
Area of a circle
A = pr2 2 p
Force of gravity
F = k/d2 -2 K
Boyle’s Law
V = k/P -1 k
Circumference of a circle varies directly as its radius. The area enclosed by a circle is directly proportional to the square of its radius. The force of gravity acting on an object is inversely proportional to the squar eo fthe distance from the object to the center of the Earth. The volume of an enclosed gas (at a constant temperature) variables inversely as the applied pressure.

5. Power Functions with Modeling
Consider this…
What happens to the circumference of a circle as its radius increases?
What happens to the area of a circle as its radius increases?
What happens to the force exerted on an object as the square of the distance from the Earth’s center increases?
What happens to the volume of gas inside an object as the pressure exerted on the object increases?

6. Power Functions with Modeling
Direct or Inverse?
Direct/Inverse
Name
Formula
Power
Constant of Variation
Circumference
C = 2pr 1 2p
Area of a circle
A = pr2 2 p
Force of gravity
F = k/d2 -2 k
Boyle’s Law
V = k/P -1

7. Power Functions with Modeling
Direct variation
Two quantities are vary directly if as one quantity increases, the other increases or as one quantity decreases, the other decreases as well.
In other words, quantities that are directly related must do the SAME things (both increase or both decrease)
Power functions f(x) = kxn that are direct variations have positive powers of n.

8. Inverse variation
Two quantities vary inversely if as one quantity increases, the other decreases or as one quantity decreases, the other increases.
In other words, quantities that are inversely related must do OPPOSITE things (one increases while the other decreases).
Power functions f(x) = kxn that are inverse variations have negative powers of n.

9. Writing a power function
Ex1 The area A of an equilateral triangle varies directly as the square of the length s of its side. Express the area as a power function in terms of s.
Does anyone know the value of the constant of variation, k?

10. Do Now
Which of the twelve basic functions we discussed are power functions? State the power of each.

11. Ex 2 State the power and constant of variation for the function, graph it, and analyze it. (domain, range, continuity, increasing/decreasing, symmetry, boundedness, local extrema, asymptotes, and end behavior)

12. Comparing Graphs of Monomial Functions
A monomial function is a single-term polynomial function.
The power functions of the form f(x) = kxa are all monomials.
Complete the Exploration on p. 183.
Which ordered pairs do the functions have in common?

13. Ex 3 Describe how to obtain the graph of g(x) given the graph of f(x) = xn with the same power of n.
g(x) = 1.5x5
g(x) = -2x6

14. Ex 4 State the values of k and a for the function f(x)
Ex 4 State the values of k and a for the function f(x). Describe the portion of the curve that lies in Quadrant I or IV. Determine whether f is even, odd, or undefined for x < 0. Describe the rest of the curve if any. Graph the function to see whether it matches your description.
f(x) = - 4x2/3
f(x) = -x -4

15. Determining Power Functions from Data
Recall r2, the coefficient of determination, can be used to determine the appropriateness of a non-linear model.
0 < r2 < 1
If r2  1, then the model is more appropriate and is a good representation for the data
If r2  0, then the model is less appropriate and is not a good representation for the data.

16. What did we discover? *All graphs pass through (1,k)*
When k>0, graph is in Quadrant 1
When k<0, graph is in Quadrant 4

17. What else did we learn? When x<0 one of three things can happen:
f is undefined for x<0
f is an even function (symmetrical about y-axis)
f is an odd function (symmetrical about origin)

18. And lastly from the activity…
When a<0 it’s inverse variation, and the graph will be asymptotic to both axes.
When a>0 it’s direct variation, and the graph will pass through (0,0)
*Page 185 top half including pictures

19. Light Intensity Data for a 100-W Light Bulb
Ex 5 Velma and Reggie gathered the data shown in the table below from a 100-watt light bulb and a CBL unit with a light-intensity probe.
Draw a scatter plot of the data
Find a regression model.
Is the power close to the a = -2 (the theoretical value)?
Sketch the graph of the equation along with the data points.
Use the regression model to predict the light intensity at distances of 1.7m and 3.4m.
Light Intensity Data for a 100-W Light Bulb
Distance
(m)
Intensity
(W/m2)
1.0
1.5
2.0
2.5
3.0
7.95
3.53
2.01
1.27
0.90