Answers and Step-by-step explanations:
(a) We want to find the slope-intercept equation (y = mx + b), where m is the slope and b is the y-intercept. We already have two points: (2, 6200) and (7, 21200). The slope is change in y over change in x so:
m = (21200 - 6200) / (7 - 2) = 3000
Now, we have y = 3000x + b. Plug in 6200 for y and 2 for x to solve for b:
y = 3000x + b
6200 = 3000 * 2 + b
b = 200
Nat's equation is y = 3000t + 200.
(b) We want a quadratic formula in vertex form because the problem says that Kat's account maxes out at 44100 on her 25th birthday, indicating that's the vertex. Our vertex point is (20, 44100); note that it's 20 and not 25 because she starts the account at age 5, not age 0. The generic vertex form of a quadratic is:
y = a(x - h)² + k, where (h, k) is the vertex
Plug in our vertex:
y = a(x - 20)² + 44100
We also have the point (9, 32000), so put that in to find a:
32000 = a(9 - 20)² + 44100
-12100 = a * 121
a = -100
So Kat's equation is y = -100(t - 20)² + 44100.
(c) In order to find the age when they have the same amount, we set the two equations equal:
3000t + 200 = -100(t - 20)² + 44100
-100(t² - 40t + 400) + 44100 = 3000t + 200
-100t² + 4000t - 40000 + 44100 - 3000t - 200 = 0
-100t² + 1000t + 3900 = 0
t² - 10t - 39 = 0
(t - 13)(t + 3) = 0
t = 13 or t = -3 -- obviously, years can't be negative, so t = 13.
Note, however, that t is the number of years <em>after</em> Kat and Nat are 5, so we need to add 13 to 5:
13 + 5 = 18
The age is 18.
