Error using cross and not calculating correctly

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Braden
Braden 2023-3-14
评论： Braden 2023-3-14
I am trying to write the code to calculate the area of a trianlge given three vectors. I keep getting an answer that is wrong. Here's the code that I wrote:
input a=['Enter vector a:'];
input b=['Enter vector b:'];
input c=['Enter vector c:'];
A=norm(cross('c-a','b-a'));
Area=A/2
This is what I get when I plug in the following vectors:
>> TAREA
a=['Enter vector a:'][1 2 2]
b=['Enter vector b:'][3 1 4]
c=['Enter vector c:'][5 2 1]
Area =
53.4649
The area should be 5.4083. Why is the code not working properly?

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回答（2 个）

Walter Roberson
Walter Roberson 2023-3-14
Ran in:
CA = 'c-a'
CA = 'c-a'
BA = 'b-a'
BA = 'b-a'
dCA = double(CA)
dCA = 1×3
99 45 97
dBA = double(BA)
dBA = 1×3
98 45 97
cross(CA,BA)
ans = 1×3
0 -97 45
cross(dCA,dBA)
ans = 1×3
0 -97 45
From this you can see that cross('c-a','b-a') is the same as cross([99 45 97],[98 45 97]) . You are taking the cross-product of the characters ['c' '-' 'a'] and ['b' '-' 'a'] rather than the cross-product of the difference in values of variables c and a and b.
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Walter Roberson
Walter Roberson 2023-3-14
Ran in:
a='Enter vector a';
double(a)
ans = 1×14
69 110 116 101 114 32 118 101 99 116 111 114 32 97
You just created a as the vector ['E', 'n', 't', 'e', 'r', ' ', 'v', 'e', 'c', 't', 'o', 'r', ' ', 'a']
Try
a = input('Enter vector a ');
b = input('Enter vector b ');
c = input('Enter vector c ');

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Dyuman Joshi
Dyuman Joshi 2023-3-14
编辑：Dyuman Joshi 2023-3-14
Ran in:
You are using character scalars instead of using variables. The result you get are corresponding to the ascii values of the characters
a=[1 2 2];
b=[3 1 4];
c=[5 2 1];
%Using variables
Area1=norm(cross(c-a,b-a))/2
Area = 5.4083
Area2=norm(cross('c-a','b-a'))/2
A = 53.4649
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Dyuman Joshi
Dyuman Joshi 2023-3-14
If you want to ask a user for inputs, try this -
a=input('Enter vector a\n');
b=input('Enter vector b\n');
c=input('Enter vector c\n');
Area1=norm(cross(c-a,b-a))/2
When you run the above code, a prompt will ask to input the values for a, b and c. Provide the values of vectors respectively and you will get the value of Area

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