how can i find x value from given y (6th degree polynomial)

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firrou bouteflika 2021-7-9

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/875573-how-can-i-find-x-value-from-given-y-6th-degree-polynomial

评论： firrou bouteflika 2021-12-19

y = [7 12 18 27 35 39];
p1 = 3.7739e-21
p2 = -6.5969e-17
p3 = 2.2715e-13
p4 = 1.1034e-09
p5 = -4.8986e-06
p6 = 0.010416
p7 = -0.70896
y = p1*x^6 + p2*x^5+p3*x^4+p4*x^3+p5*x^2+p6*x+p7;

采纳的回答

Matt J 2021-7-9

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/875573-how-can-i-find-x-value-from-given-y-6th-degree-polynomial#answer_743693

在 MATLAB Online 中打开

x=roots([p1,p2,p3,p4,p5,p6,p7-y])

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firrou bouteflika 2021-12-19

在 MATLAB Online 中打开

i'm sorry can i just ask you another time what is this unswer, i don't understand it

x = 
0e+03 *
3101 + 1.8774i   9.3101 - 1.8774i  -4.5565 + 0.0000i   1.1038 + 1.7008i   1.1038 - 1.7008i   1.2089 + 0.0000i
2800 + 1.8478i   9.2800 - 1.8478i  -4.5894 + 0.0000i   2.1232 + 0.0000i   0.6932 + 1.8383i   0.6932 - 1.8383i
2421 + 1.8102i   9.2421 - 1.8102i  -4.6272 + 0.0000i   2.7430 + 0.0000i   0.4401 + 2.0518i   0.4401 - 2.0518i
1810 + 1.7489i   9.1810 - 1.7489i  -4.6807 + 0.0000i   3.3617 + 0.0000i   0.2187 + 2.3009i   0.2187 - 2.3009i
1215 + 1.6884i   9.1215 - 1.6884i  -4.7256 + 0.0000i   3.7931 + 0.0000i   0.0849 + 2.4753i   0.0849 - 2.4753i
0896 + 1.6555i   9.0896 - 1.6555i  -4.7472 + 0.0000i   3.9881 + 0.0000i   0.0301 + 2.5515i   0.0301 - 2.5515i

because my answer should be just a one number like 2.49e+03

i'm sorry that i have to use another screenshot to explain it whit new y values

these are the y values [16.27 27.9 41.86 62.79 81.39 90.69]

Walter Roberson 2021-12-19

在 MATLAB Online 中打开

format long g

p = [3.7769e-21, -6.6055e-17, 2.2806e-13, 1.0989e-09, -4.8887e-06, 0.010408, -0.71229];

R = roots(p)

R =

9350.21416809902 + 1915.74457506144i 9350.21416809902 - 1915.74457506144i -4499.3555970064 + 0i 1608.69399831728 + 1978.77107818079i 1608.69399831728 - 1978.77107818079i 70.7499925489447 + 0i

realroots = R(imag(R)==0)

realroots = 2×1

1.0e+00 * -4499.3555970064 70.7499925489447

syms x

poly = poly2sym(p)

poly =

fplot([poly,0], [-4750 250])

It is a degree 6 polynomial in real coefficients. There are always going to be an even number of real roots: in this case there are two real roots. You can never get just one real root for a polynomial of even order that has real coefficients.

firrou bouteflika 2021-12-19