# Upper Case Letter next to Lower Case Letter in a Equation.

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?

• 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? May 4, 2018 at 8:13
• 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.
– Eff
May 4, 2018 at 8:18
• @UnruffledST Weclome to Math.SE! Please consult this for tips on formatting your questions and answers: math.meta.stackexchange.com/questions/5020/… May 4, 2018 at 8:35
• @Eff thank you! May 4, 2018 at 8:45

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.