Article in English: In algebra, you'll start to see letters and symbols appear in your math problems, rather than just numbers. These are called variables. Variables aren't as confusing as they may first seem - they're just ways of showing numbers with unknown values. Below are just a few common examples of variables in algebra:  Letters like x, y, z, a, b, and c Greek letters like theta, or θ Note that not all symbols are unknown variables. For instance, pi, or π, is always equal to about 3.14159. As mentioned above, variables are basically just numbers with unknown values. In other words, there's some number that can go in place of the variable to make the equation work. Usually, your goal in an algebra problem is to figure out what the variable is — think of it as a "mystery number" that you're trying to discover.  For example, in the equation 2x + 3 =  11, x is our variable. This means that there's some value that goes in the place of x to make the left side of the equation equal 11. Since 2 × 4 + 3 = 11, in this case, x = 4. An easy way to start understanding variables is to replace them with question marks in algebra problems. For example, we might re-write the equation 2 + 3 + x = 9 as 2 + 3 + ? = 9. This makes it easier to understand what we're trying to do — we just need to find out what number to add to 2 + 3 = 5 to get 9. The answer is again 4, of course. If a variable appears more than once, simplify the variables. What do you do if the same variable appears more than once in the equation? Though this situation may seem tricky to solve, you can actually treat variables how you'd treat normal numbers — in other words, you can add them, subtract them, and so on as long as you only combine variables that are alike. In other words, x + x = 2x, but x + y doesn't equal 2xy.  For example, let's look at the equation 2x + 1x = 9. In this case, we can add 2x and 1x together to get 3x = 9. Since 3 x 3 = 9, we know that x = 3. Note again that you can only add the same variables together. In the equation 2x + 1y = 9, we can't...

Summary in Vietnamese:
Tìm các ký hiệu không phải là số. Xem biến số là số "chưa biết". Nếu biến số xuất hiện nhiều hơn một lần, hãy đơn giản các biến số.