Question:
Find the limit algebraically:
lim x → 2 x 3 − 8 x 2 − 4 x \lim_{{x \to 2}} \frac{{x^3 - 8}}{{x^2 - 4x}} x → 2 lim x 2 − 4 x x 3 − 8 Answer:
To find the limit algebraically, we can attempt direct substitution by plugging in x = 2 x=2 x = 2 into the expression:
lim x → 2 ( 2 ) 3 − 8 ( 2 ) 2 − 4 ( 2 ) \lim_{{x \to 2}} \frac{{(2)^3 - 8}}{{(2)^2 - 4(2)}} x → 2 lim ( 2 ) 2 − 4 ( 2 ) ( 2 ) 3 − 8 However, direct substitution results in an indeterminate form of 0 0 \frac{0}{0} 0 0 . This suggests that we need to manipulate the expression in order to simplify it and find the limit.
Factoring the numerator and denominator, we have:
lim x → 2 ( x − 2 ) ( x 2 + 2 x + 4 ) x ( x − 4 ) \lim_{{x \to 2}} \frac{{(x-2)(x^2+2x+4)}}{{x(x-4)}} x → 2 lim x ( x − 4 ) ( x − 2 ) ( x 2 + 2 x + 4 ) Now, we can cancel out the common factor of ( x − 2 ) (x-2) ( x − 2 ) :
lim x → 2 ( x − 2 ) ( x 2 + 2 x + 4 ) x ( x − 4 ) = lim x → 2 x 2 + 2 x + 4 x ( x − 4 ) \lim_{{x \to 2}} \frac{{\cancel{(x-2)}(x^2+2x+4)}}{{x(x-4)}} = \lim_{{x \to 2}} \frac{{x^2+2x+4}}{{x(x-4)}} x → 2 lim x ( x − 4 ) ( x − 2 ) ( x 2 + 2 x + 4 ) = x → 2 lim x ( x − 4 ) x 2 + 2 x + 4 We still have an indeterminate form of 0 0 \frac{0}{0} 0 0 , so we need to further simplify the expression. We can do this by factoring the quadratic in the numerator:
lim x → 2 ( x + 2 ) ( x + 2 ) x ( x − 4 ) \lim_{{x \to 2}} \frac{{(x+2)(x+2)}}{{x(x-4)}} x → 2 lim x ( x − 4 ) ( x + 2 ) ( x + 2 ) Now, we can cancel out the common factors of ( x + 2 ) (x+2) ( x + 2 ) :
lim x → 2 ( x + 2 ) ( x + 2 ) x ( x − 4 ) = lim x → 2 x + 2 x − 4 \lim_{{x \to 2}} \frac{{\cancel{(x+2)}(x+2)}}{{x(x-4)}} = \lim_{{x \to 2}} \frac{{x+2}}{{x-4}} x → 2 lim x ( x − 4 ) ( x + 2 ) ( x + 2 ) = x → 2 lim x − 4 x + 2 Finally, we can substitute x = 2 x=2 x = 2 into the simplified expression:
lim x → 2 2 + 2 2 − 4 = lim x → 2 ( − 2 ) = − 2 \lim_{{x \to 2}} \frac{{2+2}}{{2-4}} = \lim_{{x \to 2}} (-2) = \boxed{-2} x → 2 lim 2 − 4 2 + 2 = x → 2 lim ( − 2 ) = − 2 Therefore, the limit of x 3 − 8 x 2 − 4 x \frac{{x^3 - 8}}{{x^2 - 4x}} x 2 − 4 x x 3 − 8 as x x x approaches 2 2 2 is − 2 -2 − 2 .