- The word variable means something that can vary i.e., change and constant means that does not vary.
- The value of a variable is not fixed.
- Variables are denoted usually by letters of the English alphabets such as x, y, z, l, m, n, p, a etc.
- The expressions are formed by performing operations like addition, subtraction, multiplication and division on the variables and constants.
- An equation is a condition on a variable (or variables) such that two expressions in the variable (variables) have equal value.
- The value of the variable for which the equation is satisfied is called the solution or root of the equation.
- An equation remains the same if the LHS and the RHS are interchanged.
- In case of balanced equation if we

- Add the same number to both the sides,
- Subtract the same number from both the sides
- Multiply both sides by the same non-zero number or
- Divide both sides by the same non-zero number, the balance remains undisturbed.

- Transposing means moving from one side to the other.
- When a term is transposed from one side of the equation to the other side, its sign gets changed.
- Transposition of an expression can be carried out in the same way as the transposition of a term.
- To solve practical problems:

- Read the problem carefully and denote the unknown quantity by variable x, y etc.
- Form the equation according to the given conditions.
- Solve the equation i.e., find the value of the unknown quantity (variable).

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