How do you solve #x^2-3x-4>=0# using a sign chart?

1 Answer
Nov 1, 2016

Answer:

The answer is #-oo < x <=-1# and #4<= x<+oo#

Explanation:

Start by factorising
#x^2-3x-4=(x+1)(x-4)#
let #y=(x+1)(x-4)#
So we can make the sign chart
#x##color(white)(aaaa)##-oo##color(white)(aaaa)##-1##color(white)(aaaa)##4##color(white)(aaaa)##+oo#
#x+1##color(white)(aaaa)##-##color(white)(aaaaa)##+##color(white)(aaaa)##+#
#x-4##color(white)(aaaa)##-##color(white)(aaaaa)##-##color(white)(aaaa)##+#
#y##color(white)(aaaaaaaa)##+##color(white)(aaaaa)##-##color(white)(aaaa)##+#

So #x^2-3x-4>=0#
when #-oo < x <=-1# and when #4<= x<+oo#