Eduki

It is an act of making people know about some feelings or thoughts. It acts as a medium so that somebody’s thoughts can be understood by others. We recognize facial expressions, written expressions, oral expressions, etc. You may make a different face when you are in pain as compared to when you are eating delicious food. The example mentioned here is mostly facial expression but other forms of expression may be used for the same situation. 

Mathematics also uses expressions and it is done by using mathematical entities such as numbers and symbols. Any combination of numbers and symbols (acting as operators) can be informative to find out the thoughts of the writer. 

For example, when we see 100-3, we understand it is trying to say that we have to reduce 100 by 3. We can often simplify a given expression into an equivalent quantity (97 in the previous case), often known as the value of the expression. 

Lets take an example.

There are 2 senators from each state in the US Senate. As there are a total of 50 states under the American flag, how many members are there in total in the senate? 

As we can see that for each state there are two members, we have to add two 50 times or it means that we must multiply 2 by 50. That is 100 members in total in the senate. It can also be interpreted as a single member from each state will make 50 members. The final answer is just double that number. 

Apart from the operators and numbers we already know, there are letters/symbols used in the expression and they are known as variables. 

Here come the letters

Just like 9-3 means we need to deduct 3 from 9, 50-a means that we have to deduct “a” quantity from 50. One thing can be validly coming to your mind, how can a letter be subtracted from a number? The real reason is that the letter used here isn't just another letter. It's the same as deducting one numerical quantity from the other. Let's consider a situation. When we are unknown of some value that needs to be operated on; for example, you are being offered 30 cents discount on all chocolates being sold at a store and you want to know how much you need to pay for one chocolate. If the price of chocolate is 3 dollars, you may have to pay 2.7 dollars, if the chocolate is worth 4.7 dollars then you may have to pay 4.4 dollars. It can be agreed upon that we can't explain the price of every chocolate with one value or one number but whatever the marked price of the chocolate may be, you need to pay 30 cents or 0.3 dollars lesser. Thus if we had an entity that could act as a placeholder for all the values of the chocolate prices, it could help us immensely to express the value needed to pay for the chocolate. 

How can all situations be explained with one expression? 

If we substitute the unknown or varying numbers with a letter that doesn't represent one particular value then it is possible to explain the different situations with a common expression. If the marked price of one chocolate is p dollars, then the price you have to pay will be p-0.3 dollars. This expression will always be representing the same situation no matter how much the price of the chocolate may be. p is nothing but a placeholder that takes the place of the actual value of the price of the chocolate. In contrast, if it was told that you have to pay 4 dollars then that is for those chocolates only whose price is 4.3 dollars. If we use another particular value then it will be for chocolate of one or few types with some price only and not all chocolates present. That is the reason why letters are used in an expression.