How to make and plot a function with a summation?

2018 1 11
3 个回答
回答已采纳
更新时间：2018 1 12
13 次查看（30 天）

1 个投票

Goodevening everybody,
I've been trying to make and plot the following function in Matlab:
I have the values for ak in a vector (n is max 100). I've been trying with symsum function and for loops in a sorts of ways but can't get it to work.
I hope someone can help me :)

请先登录，再回答此问题。

 采纳的回答

Birdman
Birdman 2018-1-11

2 个投票

Try this:
n=100;
a=sym('a',[n+1 1]);
syms x k
SmbSum=symsum(a(k+1)*x^k,k,0,n);

7 个评论
显示 5更早的评论 隐藏 5更早的评论

Bas Helfrich
Bas Helfrich 2018-1-12
Getting the following error:
Error using sym/subsindex (line 796) Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.
For some reason it doesn't take the values out of the vector.
Birdman
Birdman 2018-1-12
Try the following approach:
n=100;
a=sym('a',[n 1]);
syms x k
eq=sym(zeros(n,1));
for i=1:n
eq(i)=a(i)*x^i;
end
eq=sum(eq)
Bas Helfrich
Bas Helfrich 2018-1-12
This seems to work, only put in the numers and it should work. Thanks for your help!
Birdman
Birdman 2018-1-12
You are welcome. If it helped, can you accept it?
Bas Helfrich
Bas Helfrich 2018-1-12
I am able to put in the a values, but not yet the x to be able to make a plot of the function.
Birdman
Birdman 2018-1-12
To replace a value with a symbolic variable, use subs command. For instance
x=subs(x,1);
or
x=subs(x,0:0.1:1);

请先登录，再进行评论。

更多回答（2 个）

Steven Lord
Steven Lord 2018-1-11

0 个投票

Do you want this to be a symbolic expression with x as the symbolic variable? Or do you have a numeric value for x and want the result of evaluating this polynomial?
In the former case, see the poly2sym function.
In the latter case, I would be very careful about doing anything with a polynomial of degree 99. You're almost begging for catastrophic cancellation. But if you are careful you could use the polyval function.

2 个评论
显示 无 隐藏 无

Bas Helfrich
Bas Helfrich 2018-1-12
Eventually I want to make a plot with this function with -8 < x < 8. So with numeric values for x. I'll try the polyval function, but I still have to define the function first...
Steven Lord
Steven Lord 2018-1-12
Keep in mind:
>> x = 8^99
x =
2.5463e+89
You're going to be adding and subtracting extremely large numbers at the edges of your ranges if you go up to a degree 99 polynomial. See the catastrophic cancellation link I included in my response.
If your polynomial is of modest degree, you already have the polynomial you need to use polyval. It takes a coefficient vector and you said "I have the values for ak [the coefficients] in a vector".
x = polyval([1 2 3], 4)
y = 1*4^2 + 2*4^1 + 3*4^0
f = @(x) x.^2 + 2*x.^1 + 3;
z = f(4)

请先登录，再进行评论。

Torsten
Torsten 2018-1-12

0 个投票

Is this the partial Taylor series of a certain function f ?
In this case, you could use
f=...;
n=...;
fn = taylor(f,x,'order',n);
fplot(fn);
xlim([-4 4])
grid on
Best wishes
Torsten.

类别

在 帮助中心 和 File Exchange 中查找有关 Calculus 的更多信息

标签

Community Treasure Hunt

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

Start Hunting!

Translated by