How can I make summation using variable symbol

clc
clear
syms p
a=[0.01;0.015];%The cofficeient next to power in the IFC equation 
b=[8.5;9.5];%The cofficeientin the IFC equation
B=[4e-4 0; 0 4e-4];
pl =[];
for i=1:length(a)
    for j=1:length(a)
        pl = pl + p(i)*B(i,j)*p(j)
    end
end

Also, can I later on differentiate the pl equation based on p(i)? so i can find dpl/dp1 or dpl/dp2 using such code

for i=1:length(a)
    dpl(i)=diff(pl,p(i))
    k(i)=1/(1-dpl(i));
end

Answer 1

[VBBV]

clc
clear
ng = 2;
syms p [1 ng] 

a=[0.01;0.015];%The cofficeient next to power in the IFC equation 
b=[8.5;9.5];%The cofficeientin the IFC equation
B=[4e-4 0; 0 4e-4];
 
for i=1:length(a)
    for j=1:length(a)
        pl(j,i) = p(i)*B(i,j)*p(j);
    end
    Pl(i) = sum(pl(:,i));
end
Pl = sum(Pl)

Pl = 

dp1=diff(Pl,p1); % differentiate with p1
dp2=diff(Pl,p2);  % differentiate with p2
Dp = [dp1 dp2];
k=1./(1-Dp)

k = 

[Osama Aliwat]

Thank you alot. For the differentiation part it will only find dp1 and dp2 while i need it to find dp3 or dp10 depending on the matrix of a thats why I used for loop