Find the following limit:
lim_(x->∞) (sqrt(9 x^2+x)-3 x)
sqrt(9 x^2+x)-3 x = (3 x+sqrt(9 x^2+x))/(3 x+sqrt(9 x^2+x)) (sqrt(9 x^2+x)-3 x) = x/(3 x+sqrt(9 x^2+x)):
lim_(x->∞) x/(3 x+sqrt(9 x^2+x))
Write x/(3 x+sqrt(9 x^2+x)) as 1/(3+sqrt(9 x^2+x)/x):
lim_(x->∞) 1/(3+sqrt(9 x^2+x)/x)
Using the reciprocal rule, write lim_(x->∞) 1/(3+sqrt(9 x^2+x)/x) as 1/(lim_(x->∞) (3+sqrt(9 x^2+x)/x)):
1/(lim_(x->∞) (3+sqrt(9 x^2+x)/x))
lim_(x->∞) (3+sqrt(9 x^2+x)/x) = lim_(x->∞) 3+lim_(x->∞) sqrt(9 x^2+x)/x:
1/(lim_(x->∞) 3+lim_(x->∞) sqrt(9 x^2+x)/x)
Since 3 is constant, lim_(x->∞) 3 = 3:
1/(3+lim_(x->∞) sqrt(9 x^2+x)/x)
Simplify radicals, sqrt(9 x^2+x)/x = sqrt((9 x^2+x)/x^2):
1/(3+lim_(x->∞) sqrt((9 x^2+x)/x^2))
Using the power rule, write lim_(x->∞) sqrt((9 x^2+x)/x^2) as sqrt(lim_(x->∞) (9 x^2+x)/x^2):
1/(3+sqrt(lim_(x->∞) (9 x^2+x)/x^2))
The leading term in the denominator of (9 x^2+x)/x^2 is x^2. Divide the numerator and denominator by this:
1/(3+sqrt(lim_(x->∞) (9+1/x)/1))
The expression 1/x tends to zero as x approaches ∞:
1/(3+sqrt(9))
1/(3+sqrt(9)) = 1/6:
Answer: |1/6