Problem of summation-mathematica

[shafieza_garl]

I need help.im having problem to differentiate my equation respect to the m_i;How can i differentiate the equation.
Second question is;When i set Y=2;how can i put the parameter like m_1;m_2;S_1;S_2;
what i just do is input it manually by changing m_1=m1;m_2=m2;
its very difficult if Y become larger number

Sum[(Subscript[\[Sigma], i] - Subscript[c, i])*(Subscript[q, i]/
Subscript[T, i]), {i, 1, Y}] -
Sum[((1 - \[Beta])/(2 - \[Beta]))*(Subscript[d, i]*
Subscript[q, i]) +
(1/2)*((Subscript[m, i] - 1)*Subscript[w, i]*Subscript[q, i]) +
(Subscript[A, i]/Subscript[m, i] + Subscript[S, i])/
Subscript[T, i], {i, 1, Y}]

Thanx for da help

[Bill Simpson]

Y=2;
s=Sum[...];
d=D[s,Subscript[m,1]];
d/.Subscript[m,1]->4

[shafieza_garl]

Thanx a alot for da help.but what is da function of
d /. Subscript[m, 1] -> 4
and do you know how can input the subscript like m_1 and m_2 if they have value ?

[Bill Simpson]

expression/.variable->value
is an alternate way of writing the ReplaceAll function.
That function replaces all variable with value in expression.
Here is the documentation for ReplaceAll.
http://reference.wolfram.com/mathematica/ref/ReplaceAll.html
If you look at the value of the variable d in my code and compare that
with the value after the ReplaceAll you can see what it has done.

I am not certain I understand what you mean "if they have value."
If they have value then they are not a variable in your expression
and there is no variable to replace with a value.