x^2-x-20=0 factor please? is the answer (x-5)(x+4) its been awhile

Question

I can't remember how to factor. Do I need to use quadratic formula for this or can I just easily factor it? What method should I use to solve this? I don't remember how to do basic factoring...

Answer 1

x^2-x-20=0

 if X^2  + aX + b = 0

then ;

product of roots is b

sum of roots is a

there you have to look for numbers whose sum is a and product is b

in the equation we have

x^2-x-20=0

it can be easily seen that

-5 + 4 = -1

-5 * 4 = -20

then we shall have to expand  the equation

we have X^2 +4X -5X -20 = 0

 X(X+4) -5(X+4) = 0    from (X^2 +4X) +(-5X -20)

then (X-5)(X+4)=0