Question

factorise x^2-4x-21

Answer 1

Use the product and sum method to factor quadratics.
Find two number that multiply to get -21 and add to get -4

Answer 2

or you can simple use \[x _{1,2}=\frac{ -b \pm \sqrt{b ^{2}-4ac} }{ 2a }\]

Answer 3

Source http://www.mathsisfun.com/algebra/polynomials-sums-products-roots.html

Answer 4

Yes, you can use the quadratic formula, but sometimes it is too tedious, so a faster approach would the product and sum method.

Answer 5

(x-7) (x+3)

Answer 6

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Answer 7

\(\Large \begin{align} \color{black}{\text{u can also use squaring method}\\
x^2-4x-21\\
=x^2-4x+4-4-21\\
=(x-2)^2-25\\
=(x-2)^2-(5)^2\\
=(x-2+5)(x-2-5)\\
=(x+3)(x-7)}\end{align}\)