How to determine algorithm complexity?

I can't seem to understand how to determine the complexity of algorithms.

For example:

for j=n:-1:1
    for i=j+1:n
        x(i,j)=0
    x(j,j)=b(j,j)/c(j,j)
    for i=j-1:-1:1
         x(i,j)=(b(i,j)-c(i,i+1)*x(i+1,j))/c(开发者_如何学运维i,i)

This is more of a math problem, but still.

I use simple sum formulas and I find the result to be 2·n² but it seems the correct result is 5·n²/2.

Can someone please help me understand the correct way of calculating this?

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Assuming you are writing in MATLAB, the complexity is O(n^2). Observe that you are evaluating the sum:

sum_{j=1 to n} j + (n - j) = sum_{j=1 to n) n = n^2