Factoring quadratic trinomials

Answer 1

To factor quadratic trinomials, we find the two numbers which add to a certain number, and multiply to equal a certain number. Let me explain using the general form of a quadratic trinomial: ax2 + bx + c.

If the quadratic trinomial is monic (coefficient on x2 is 1), then you simply find two numbers which add up to the coefficient of x (b) and multiply to give the constant (c).

For example: The monic quadratic trinomial x2 - 5x + 6. Two numbers which add up to to give -5 and multiply to give 6 are -3 and -2. (-3 + -2 = -5; -3 x -2 = 6). Therefore the factors are (x - 3)(x-2).

If the quadratic trinomial is non-monic (coefficient on x2 is greater than 1), then there's a slightly different method to go around solving them. I'll show this through the example: 2x2 + 5x - 3. We start off by changing the coefficient by multiplying it by the coefficient of x2 (in this case, 2). So the new coefficient will by -6. Then we split the equation into two factors (2x )(2x ) [They are blank because we don't know the numbers yet], and divide the factors by the coefficient of x2 (which is 2). So we have (2x )(2x ) divided by 2.

Then we use the method of solving monic quadratics, and find two numbers which add to give +5, and multiply to give -6 (the new constant). In this case, the numbers are +6 and -1. (6 + -1 = 5; 6 x -1 = -6). So we put in our two numbers to get (2x + 6)(2x - 1) all over 2. Lastly, we make sure we factorise any of the factors [(2x + 6) can be factored as 2(x + 3)] to get 2(x+3)(2x-1) all over 2. And we just cancel out both twos on the numerator and denominator, and we are left with (x + 3)(2x - 1).

One last example for non-monic quadratics: 4x2 + 7x - 2. First step: Multiply coefficient of x2 with constant to get new constant (4 x -2 = -8). Then we put the coefficient of x2 over the two factors we are yet to find. So we get (4x )(4x ) divided by 4. We then find the two numbers which add to give 7 and multiply to give -8, which are +8 and -1. We then have (4x + 8)(4x - 1) all over 4. Factorise whatever we can, to get 4(x+2)(4x-1) all over 4. Cancel out the 4s and we are left with (x + 2)(4x - 1).