Yes I used the search function.

Let us suppose we have two simple supply and demand equations

$$Q_d = 20 – 2P$$ $$Q_s = -10 + 2P$$ To find where $Q_S = Q_d$ we put the two equations together

$$20-2P = -10 + 2P$$ $$20+10= 4P$$ $$\frac{30}{4}=P$$ $$P = 7.5$$ To find $Q$, we just put this value of $P$ into one of the equations

$$Q = 20 – (2×7.5)$$ $$Q= 5$$

in the example above it defines $Q_s$ and $Q_d$ now to my understanding that would mean quantity of demand and supply still is there a rule to this? is putting a Capital next to a Lower Case just a way to define two words together? Like saying having a $Q$ + $q$ means two different quantities still would putting $Q_f$ and $Q_s$ suffice also as saying first quantity and second quantity?

  • $\begingroup$ Please type out the equations with MathJax. Currently it's not very clear. Are the lower case letters subindices or multiplication, or even just variable names with more than one letter? $\endgroup$
    – Matti P.
    May 4, 2018 at 8:13
  • 3
    $\begingroup$ Usually, you wouldn't just put it as lower case next to the upper case. You would use subscript, i.e. $Q_d$ and not write $Qd$. This is done to indicate one variable instead of two. $\endgroup$
    – Eff
    May 4, 2018 at 8:18
  • $\begingroup$ @UnruffledST Weclome to Math.SE! Please consult this for tips on formatting your questions and answers: math.meta.stackexchange.com/questions/5020/… $\endgroup$ May 4, 2018 at 8:35
  • $\begingroup$ @Eff thank you! $\endgroup$
    – RebornXD
    May 4, 2018 at 8:45

1 Answer 1


In many instances, like this one here, you'll have a set of variables which are all very similar, and so instead of labelling them ${A,B,C,D...}$, we would instead use ${x_A, x_B, x_C, x_D...}$ or ${x_1,x_2, x_3, x_4...}$. In this case, $Q$ means quantity, so $Q_s$ simply means "Quantity of Supply", while $Q_d$ means "Quantity of Demand". Since they're both quantities, it makes better sense to link them together.


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