How can I change the numbers to negative without altering the positive ones and make a sum of that? This is the serie 1-2+3-5+8-13+21-34+...

n=8;

F=[1 2];

suma=0;

for i=3:n

F(i)=F(i-1) + F(i-2);

end

disp(F)

this is only for show the positive serie

Answer 1

[Voss]

n=8;
F=[1 2];  
% suma=0;
for i=3:n
    F(i)=F(i-1) + F(i-2);
end
disp(F)

     1     2     3     5     8    13    21    34

F(2:2:end) = -F(2:2:end)

F = 1×8

1 -2 3 -5 8 -13 21 -34

cumsum(F)

ans = 1×8

1 -1 2 -3 5 -8 13 -21

sum(F)

ans = -21