Factorise 2*x^2-(9*x)+4

2*x^2-(9*x)+4

$$\\2x^2-9x+4\\\\$$

 

You need to find 2 numbers that multiply to 2*4=8  and add togeter to give -9

That is easy  -8 and -1

Now split -9x into  -8x - 1x

like this

 

$$\\2x^2-8x-1x+4\\\\$$

now group them in pairs like this

$$\\2x(x-4)-1(x-4)\\\\$$

 

Now you have 2x lots of (x-4) take away 1 lot of (x-4)

that is 

$$\\(2x-1)(x-4)\\\\$$

2x² - 9x + 4 = (2x - 1)(x - 4)

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