thegame12 wrote:

Hello,

Steep 1 : x²-5x-6=0

Steep 2 : (x-6)(x+1)

How should we make the transition? I don't get it

Thanks!

You want to find two numbers - let's call them A and B - that have the following properties:

- when you multiply them together, you get the last number in the quadratic (in this case, -6. Be careful about the negative sign!)

- when you add them together, you get the middle number in the quadratic (in this case, -5.)

So, what two numbers add together to make -5, and multiply together to make -6?

The easiest place to start is to think about which numbers multiply to -6, then try adding them together and see what happens.

For instance, -3*2 = -6, but -3+2 doesn't equal -5. So that isn't the right pair of numbers.

But, -6*1 = -6, and -6+1 = -5. That's the right pair of numbers.

Once you have those numbers, you can write the quadratic like this:

(x-6)(x+1) = 0

Again, be careful to keep the negative signs the same as what you figured out earlier!

Finally, two things to be careful about:

- you can only use this technique when the first term in the quadratic doesn't have a coefficient. In other words, you can only do this when the quadratic starts with just \(x^2\) (or any variable squared), not when it starts with something like \(2x^2\). If it does, you have to divide the whole quadratic to make the 2 go away first.

- this technique is only useful when the quadratic is equal to 0. That is, when there's a 0 on the other side of the equals sign. The whole point of doing this is to find the values of x, and it's only easy to do that when you know that the product comes out to 0.

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