First let's find what x values make the expression equal to zero.
2x2 - x - 1 = 0
Divide through by 2.
x2 - (1/2)x - (1/2) = 0
What two numbers add to -1/2 and multiply to -1/2 ? → -1 and 1/2
So we can factor this
(x - 1)(x + 1/2) = 0
Set each factor equal to zero and solve for x.
x - 1 = 0 x + 1/2 = 0
x = 1 x = -1/2
So, the interval that makes the expression less than zero will be
either ( - ∞ , -1/2 ) \(\cup\) ( 1 , ∞ ) or ( -1/2 , 1 )
Test a point in the first interval, say -1 .
2(-1)2 - (-1) - 1 < 0
2 < 0 false
Test a point in the second interval, say 0 .
2(0)2 - 0 - 1 < 0
-1 < 0 true
So, the x-values which make the expression less than zero must be in the interval ( -1/2 , 1 )