Pearson Algebra 2 Common Core, 2011
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Pearson Algebra 2 Common Core, 2011 View details
2. Standard Form of a Quadratic Function
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Exercise 48 Page 207

Practice makes perfect
a We are given the two functions below. One is linear, and the other is a quadratic function.
f(x)=4x+3 g(x) = 1/2x^2+2 We have to find the rate of change of each function from x=0 to x=1. Before starting the computations, let's remember the expression that gives us the rate of change of a function F(x) between x=a and x=b.

Rate of Change &= Change iny/Change in x ⇕ Rate of Change&= F(b)-F(a)/b-a Using this formula, we can find the rate of change of each of the given functions.

Rate of Change = F(b)-F(a)/b-a
f(1)- f(0)/1-0 g(1)- g(0)/1-0
4(1)+3-(4(0)+3)/1-0 12(1)^2+2-( 12(0)^2+2)/1-0
4 0.5

From the table above, we conclude that the function f(x) has a greater rate of change from x=0 to x=1. We can also check it graphically.

Looking at the graph, we can say that both functions have the same change in the x-coordinates, but the change in the y-coordinates is greater for the function f(x). Therefore, f(x) has a greater rate of change.

b First, let's remember the formula to calculate the rate of change of a function F(x) from x=a to x=b.
Rate of Change &= Change iny/Change in x ⇕ Rate of Change&= F(b)-F(a)/b-a

As in Part A, let's find the rate of change of each of the given functions from x=2 and x=3.

Rate of Change = F(b)-F(a)/b-a
f(3)- f(2)/3-2 g(3)- g(2)/3-2
4(3)+3-(4(2)+3)/3-2 12(3)^2+2-( 12(2)^2+2)/3-2
4 2.5

From the table above, we conclude once more that the function f(x) has a greater rate of change from x=2 to x=3. We can also check it graphically.

Looking at the graph, we can say that both functions have the same change in x-coordinates, but the change in y-coordinates is greater for the function f(x). Therefore, f(x) has a greater rate of change.

c From the two previous results, and since the graph of y= f(x) is a line, we can affirm that the rate of change of f(x) is always constant and equal to 4. However, the rate of change of g(x) is not constant, and it increases as x increases.
Rate of Change = g(b)- g(a)/b-a, a< bWe are interested in finding, if any, the values of a and b for which the expression above is greater than 4.
g(b)-g(a)/b-a > 4
( 12b^2+2) - ( 12a^2+2 )/b-a > 4
â–Ľ
Simplify
12b^2+2 - 12a^2-2/b-a > 4
12b^2 - 12a^2/b-a > 4
b^2-a^22/b-a > 4
b^2-a^2/2(b-a) > 4
(b-a)(b+a)/2(b-a) > 4
b+a/2 > 4
b+a > 8
Let's analyze the inequality a+b> 8 depending on the value of a.
  1. If a< 4, it will depend on the value of b to achieve this. For example, for a=3 and b=5.5, we will have a+b> 8, but if b=4.5 then a+b < 8.
  2. If a≥ 4, then for all b> 4 we will have a+b> 8.

We conclude that a must be greater than or equal to 4. Therefore, the rate of change of g(x) is greater than 4 for all x≥ 4.