# What is the vertex form of #y= - x^2 - 10x + 20 #?

##### 2 Answers

#### Explanation:

Vertex form of a parabola:

In order to put a parabola into vertex form, use the complete the square method.

Add the value that will cause the portion in parentheses to be a perfect square.

Since we added

Notice that the

This is the equation in standard form. It also tells you that the vertex of the parabola is

#### Explanation:

By using the vertex form (completing the square) you introduce an error. If this error is '+some value' then you correct by including '- the same value'

Given:

Consider just the right hand side

write as

,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Write instead :

Multiplying

Using this to replace the brackets in expression (2)

We have gained the extra value of

so it is

However, it

Giving the final answer of

'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~