Why the result does change when ı add other rows in Matrix?

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Merve Ala
Merve Ala 2023-1-13
评论： Steven Lord 2023-1-13
I am trying basic multiplication on Matlab. For the first row, it calculates in true way. But when ı add other rows, it calculates first row wrong.
l_1=600
A= 48.6
K = [12, 6*(l_1), -12 , 6*(l_1) ;
6*(l_1), 4*(l_1)^2 , -6*(l_1), 2*(l_1)^2 ;
-12, -6*(l_1), (12), -6*(l_1) ;
6*(l_1), 2*(l_1)^2, -6*(l_1), 4*(l_1)^2 ]
K_result=K.*A
and the result is
K_result =
1.0e+07 *
0.0001 0.0175 -0.0001 0.0175
0.0175 7.0000 -0.0175 3.5000
-0.0001 -0.0175 0.0001 -0.0175
0.0175 3.5000 -0.0175 7.0000
it doesnt calculate A*12 the result must be 583.3 but it shows 0.0001. When i just calculate first row it calculates in true.
K = [12, 6*(l_1), -(12), 6*(l_1)]
K_result=K.*A
K_result =
1.0e+05 *
0.0058 1.7500 -0.0058 1.7500

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采纳的回答

Kevin Holly
Kevin Holly 2023-1-13
Ran in:
It is giving the correct result. It is just rounded. I changed format to long below.
l_1=600;
A= 48.6;
K = [12, 6*(l_1), -12 , 6*(l_1) ;
6*(l_1), 4*(l_1)^2 , -6*(l_1), 2*(l_1)^2 ;
-12, -6*(l_1), (12), -6*(l_1) ;
6*(l_1), 2*(l_1)^2, -6*(l_1), 4*(l_1)^2 ];
K_result=K.*A
K_result = 4×4
1.0e+07 * 0.0001 0.0175 -0.0001 0.0175 0.0175 6.9984 -0.0175 3.4992 -0.0001 -0.0175 0.0001 -0.0175 0.0175 3.4992 -0.0175 6.9984
format long
K_result=K.*A
K_result = 4×4
1.0e+07 * 0.000058320000000 0.017496000000000 -0.000058320000000 0.017496000000000 0.017496000000000 6.998400000000000 -0.017496000000000 3.499200000000000 -0.000058320000000 -0.017496000000000 0.000058320000000 -0.017496000000000 0.017496000000000 3.499200000000000 -0.017496000000000 6.998400000000000
  2 个评论
Steven Lord
Steven Lord 2023-1-13
Ran in:
In this case IMO format longg is even better.
l_1=600;
A= 48.6;
K = [12, 6*(l_1), -12 , 6*(l_1) ;
6*(l_1), 4*(l_1)^2 , -6*(l_1), 2*(l_1)^2 ;
-12, -6*(l_1), (12), -6*(l_1) ;
6*(l_1), 2*(l_1)^2, -6*(l_1), 4*(l_1)^2 ];
format longg
K_result=K.*A
K_result = 4×4
1.0e+00 * 583.2 174960 -583.2 174960 174960 69984000 -174960 34992000 -583.2 -174960 583.2 -174960 174960 34992000 -174960 69984000

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