How do you solve #x^2 + 4x - 21 = 0#?
We are going to solve it with
The equation is of the form
You can solve this by using the general formula for complete quadratic equations.
For a complete quadratic equation
Let's apply it to your equation, which gives:
So, we've got two solutions:
A less direct, but perhaps simpler approach is to recognise that if
The reason I say it must look like this with one
What pair of factors of
There are not many choices of factorizations, basically either
This is zero when