Quadratic Expressions.....

An expression has no equal sign. A basic algebraic expression is made up of algebraic terms together with numbers such as : x + 3 , z + 5. It is normally made up of a variable and a number. A standard algebraic equation would be by + c.
Whereas, a quadratic expression is made of up algebraic terms of two different powers together with numbers. Examples would be : x^2 + 7y +2 , 3x^2 + 2y + 6. A standard quadratic equation is : ax^2 + by + c.  
A quadratic equation is different from an algebraic expression as it has " ax^2 " that does not exist in algebraic expressions. Thus, in an quadratic expression, a cannot be equals to 0 or it will be an algebraic expression.