Urgent help needed

im in a bit of a rush so sorry if this is the wrong section to ask but can someone pls help me with this quesion

Factorise 15xsquared -3x-18

i know the anwser but i still cannot seem to understand how factorsing these epressions work , if anyone could be a detailed explanation then that would be very helpful

Reply 1

RogerOxon

21

Original post by sirhazthe1s

im in a bit of a rush so sorry if this is the wrong section to ask but can someone pls help me with this quesion

Factorise 15xsquared -3x-18

i know the anwser but i still cannot seem to understand how factorsing these epressions work , if anyone could be a detailed explanation then that would be very helpful

15x^2-3x-18

(3x+a)(5x+b)

You need a,b s.t.

ab=-18

(equating the constant),

3b+5a=-3

(equating the x coefficient)

Obvious number to try are factors of -18, e.g. (make one or the other negative) (1,18), (2,9), (3,6). These satisfy the

ab=-18

. Which satisfy

3b+5a=-3

?

Hint: Start with

\pm{3}

and

\mp{6}

, as -3 is fairly small.

(edited 4 years ago)

Reply 2

RogerOxon

21

Original post by sirhazthe1s

im in a bit of a rush so sorry if this is the wrong section to ask but can someone pls help me with this quesion

Factorise 15xsquared -3x-18

i know the anwser but i still cannot seem to understand how factorsing these epressions work , if anyone could be a detailed explanation then that would be very helpful