Converting Quadratics: Standard Form and Vertex Form

Key concept

Standard form and vertex form are two ways to write the same quadratic. Standard form is y = ax² + bx + c and vertex form is y = a(x - h)² + k. Change between them by completing the square or expanding the brackets.

Converting Quadratics: Standard Form and Vertex Form - introduction visual

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Converting Quadratics: Standard Form and Vertex Form poster

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Flashcards

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Converting quadratic equations between general form y = ax² + bx + c and vertex form y = a(x - h)² +  k, where a ≠ 0.Converting a quadratic equation from vertex form, y = 2(x - 3)² - 5, to general form by expanding and simplifying to y = 2x² - 12x + 13.Conversion of a quadratic equation from general form y = 3x² + 18x + 25 to vertex form by completing the square, resulting in y = 3(x + 3)² - 2.Converting the quadratic equation y = -2x² + 8x - 5 from general to vertex form using completing the square method, resulting in y = -2(x - 2)² + 3.

What are Standard Form and Vertex Form?

  • Standard (general) form:
  • Vertex form:

Converting Vertex Form to Standard Form

  • Expand the square using , then expand the brackets
  • Don't forget the constant at the end (this is a very common mistake)

Converting Standard Form to Vertex Form

  • Only factor out from the and terms. Ignore the constant
  • E.g.

Completing the Square

  • When completing the square, anything added inside must be undone outside
  • E.g.

Practice Questions

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Q1Easy

Convert the quadratic equation into general form.

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General form writes a quadratic equation as three separate terms added together. Vertex form keeps one squared binomial multiplied by a coefficient with a constant on the end. Both layouts describe the same equation, so converting between them does not change the underlying quadratic.

Square the binomial inside the brackets using the squared binomial formula. Multiply every term inside the brackets by the coefficient outside. Add the constant that sits at the end of the expression. Then simplify by collecting like terms. The result is the equation in general form.

Factorise out the coefficient of x squared from the x squared and x terms. Take the same sign and half the coefficient of x to form a new binomial. Square the new binomial and subtract the square of the halved number. Then simplify the expression.

Squaring the new binomial creates an extra constant term that was not part of the original expression. You then subtract the square of the halved number so the value of the expression stays the same. The converted equation still equals the original quadratic equation.

Yes, the method always works for any quadratic equation. The coefficient of x squared can be positive, negative, or any value other than one. The worked examples in this lesson use coefficients of 2 and 3. The same four steps apply to both.

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