# find the coefficients a, b, and c of the quadratic polynomial. that passes through the three points (x, y)

조회 수: 194(최근 30일)
Sabrina Lozano 2022년 4월 12일
댓글: Sabrina Lozano 2022년 4월 12일
find the coefficients a, b, and c of the quadratic polynomial y = ax^2+ bx + c that passes through the three points (x, y) = (1, 4), (4, 73), (5, 120)
I am not sure how to go code this problem and need help!!

댓글을 달려면 로그인하십시오.

### 답변(4개)

KSSV 2022년 4월 12일
% Convert the equation to form Ax = b
P = [1, 4 ;4, 73; 5, 120] ; % points
x = P(:,1) ;
y = P(:,2) ;
b = P(:,2) ; % RHS
A = [x.^2 x repelem(1,3,1)] ;
% solve
x = A\b ;
% check
A*x
ans = 3×1
4.0000 73.0000 120.0000
##### 댓글 수: 1표시숨기기 없음
Sabrina Lozano 2022년 4월 12일
What does the A=[x.^2 x repelem[(1,3,1)]; do?

댓글을 달려면 로그인하십시오.

KALYAN ACHARJYA 2022년 4월 12일
편집: KALYAN ACHARJYA 2022년 4월 12일
Sufficients Hints:
As it is the part of your work, please check the hints below, the equation is y = ax^2+ bx + c with three given Points.
Step 1:
Subsititute all those three points in the eqaution, you will get the three eqaution with unknown a,b,c
Step 2:
Create an Augumented Matrix using step 1 (3 equations)
Step 3:
Solve it for a,b,c
Do it yourself is the best way of learning!
Hope it helps!
##### 댓글 수: 3표시숨기기 이전 댓글 수: 2
Sabrina Lozano 2022년 4월 12일
This code was for a cubic polynomial

댓글을 달려면 로그인하십시오.

VBBV 2022년 4월 12일
syms a b c
x = [1 4 5];
y = [4 73 120];
eqn = y == a*x.^2+b*x+c
eqn =
sol = solve(eqn,[a b c])
sol = struct with fields:
a: 6 b: -7 c: 5
##### 댓글 수: 4표시숨기기 이전 댓글 수: 3
Sabrina Lozano 2022년 4월 12일
ah okay I only have the signal processing tool box.

댓글을 달려면 로그인하십시오.

Sam Chak 2022년 4월 12일
편집: Sam Chak 2022년 4월 12일
This problem can be rather easily solved with Curve-Fitting Toolbox with a click of a button or one simple function:
p = polyfit(x, y, 2)
However, I believe this is not the result your Professor wants to see. She or he probably wants to look at your efforts in computing the code according to what is taught in her or his lecture.
I'd suggest you to use a spreadsheet to tabulate out the necessary computations to be performed on the three points
Then, compose this:
which exactly a linear system
.
The solution is given by
which is easily computed in MATLAB without using the Curve-Fitting Toolbox
x = A\b
But you may lose some marks for not computing the inverse of matrix A manually that involves calculating the determinant. Better check with your Professor if computing the matrix inverse is necessary shown in the working.
Don't worry about being discovered by your Professor as these are merely guidelines from the textbook or public domain knowledge. You just have to compute the required items as shown above.
Hope this explanation is helpful for you to discover your learning experience as mentioned @KALYAN ACHARJYA.
##### 댓글 수: 1표시숨기기 없음
Sabrina Lozano 2022년 4월 12일
Thank you very much this explanation was very helpfull !!! This is my fisrt class using matlab im still tryin to learn the basics!

댓글을 달려면 로그인하십시오.

### 범주

Find more on Conversion Between Symbolic and Numeric in Help Center and File Exchange

R2021b

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by