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Identify the factors of x2 - 4x - 21.
Well can you think of 2 numbers, one positive and one negative, that will add to -4 and multiply to -21?
Give me a sec
And yeah, that is x^2-4x-21
I can't think of one besides 5 and -9. But when multiplied that will equal -45 so thats not it
Don't focus on the "add to -4" part first. Think of the "multiply to -21" first. What pairs of numbers can you multiply to get 21?
x^2-7x+3x-21 x(x-7)+3(x-7) (x+3)(x-7) are the factors
Thanks Pawanyadav but I want to learn how to do this too.
Oh, would it be -7 and 3?
Yeah, it'd be -7 and 3. Once you know that, all you have to do is put it together \[\large (x-7)(x+3)\]
Oh, okay. Seems pretty simple. Will these kind of questions always be like this? Like, do I always have to find what you multiply and add to get that certain number?
And @Pawanyadav I'll give you a fan since you weren't incorrect, and Sadist a medal since he walked me through it.
Break -4 in such a way so that the two break parts has the product -21 and definitely the parts are broken from -4 they will give their sum equal to-4.
Well it'll definitely get harder once they get more complicated. I like this method as a beginning method though, since it give a good understanding of how to do it and you can always fall back to it.
Do you understand now?
Alright, I understand now. Thanks you two.