# Linear algebra population question, fit second degree polynomial through 3 points?

A country s census lists the population of the country as 250 million in 1990, 284 million in 2000, and 310 million in 2010. Fit a second-degree polynomial passing through these three points. (Let the year 2000 be x = 0 and let p(x) represent the population in millions.)

P(x) =

2020

### 3 Answers

- davidLv 74 weeks ago
A country s census lists the population of the country as 250 million in 1990, 284 million in 2000, and 310 million in 2010. Fit a second-degree polynomial passing through these three points. (Let the year 2000 be x = 0 and let p(x) represent the population in millions.)

(0, 284) ... (10,310) ... (-10, 250)

y = ax^2 + bx + c

284 = a(0) + b(0) +c .... c = 284

310 = a(10^2) + b(10) + 284

26 = 100a + 10b

=============================

250 = a(-10)^2 + b(-10) + 284

-34 = 100a - 10b

-[26 = 100a + 10b]

-8 = -20b ... b= 0.4

100a = 26 - 10(0.4)

a = 0.22

============

p(x) = 0.22x^2 + 0.4b + 284

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- alexLv 74 weeks ago
P(x) = ax^2+bx+c

points (0,284) , (-10 , 250) , (10 , 310)

set up 3 equations then solve for a , b , c

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- llafferLv 74 weeks ago
If x is the number of years after 2000, then the year 1990 is when x = -10, etc.

You are given three points:

(-10, 250), (0, 284), and (10, 310)

Where "y" is in millions.

You know it's a quadratic, so we can start from that general form:

y = ax² + bx + c

We are given three points which is three sets of x's and y's. Substitute each set into the general form to get a system of three equations and three unknowns:

250 = a(-10)² + b(-10) + c and 284 = a(0)² + b(0) + c and 310 = a(10)² + b(10) + c

250 = 100a - 10b + c and 284 = 0a + 0b + c and 310 = 100a + 10b + c

250 = 100a - 10b + c and 284 = c and 310 = 100a + 10b + c

we have a value for c. Substitute into the other two to make a new system of two equations and two unknowns:

250 = 100a - 10b + 284 and 310 = 100a + 10b + 284

-34 = 100a - 10b and 26 = 100a + 10b

Since b's coefficients are opposites, I'll solve them with elimination. Add the two equations together to get:

-8 = 200a

a = -8/200

a = -1/25

Now that we have a, we can solve for b:

-34 = 100a - 10b

-34 = 100(-1/25) - 10b

-34 = -4 - 10b

-30 = 10b

b = 3

Your equation is:

y = (-1/25)x² + 3x + 284

Where x is the number of years after 2000 and y is the population in millions.

- PuzzlingLv 74 weeks agoReport
I concur with the method and the solution. Here's a graph of the solution: https://www.desmos.com/calculator/stggad3jvo

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