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Lesson 5. Limits at Infinity

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LESSON 5.

LIMITS at INFINITY

LIMITS AT INFINITY
THEOREM 1:
Let r be any positive integer. Then,
1 1
a. lim =0 b. lim =0
𝑥→+∞ 𝑥𝑟 𝑥→−∞ 𝑥 𝑟

NOTE: When evaluating the limit of a quotient at infinity, first divide both numerator and
denominator by the highest power of the variable appearing in the denominator.
EXAMPLES:
Evaluate
1
1. lim =0
𝑥→+∞ 𝑥 8

12
2. lim = 12 • 0 = 0
𝑥→+∞ 𝑥 15

2𝑥 3 3
2𝑥+3 + 2+ 2+0 2
𝑥 𝑥 𝑥
3. lim = lim 4 𝑥 = lim 4 = 0−1 = −1 = −2
𝑥→+∞ 4−𝑥 𝑥→+∞ 𝑥−𝑥 𝑥→+∞ 𝑥−1

𝑥2 2𝑥 1 1 2 1
𝑥 2 +2𝑥+1 + + + + 0+0+0 0
𝑥3 𝑥3 𝑥3 𝑥 𝑥2 𝑥3
4. lim = lim 3 = lim 4 = =1=0
𝑥→−∞ 𝑥 3 +4 𝑥→−∞ 𝑥 + 4 𝑥→−∞ 1+ 3 1+0
𝑥3 𝑥3 𝑥

𝑥
𝑥 𝑥 1 1 1
5. lim = lim = lim = =1=1
𝑥→+∞ √𝑥 2 −1 𝑥→+∞ 𝑥2 1
√ 2− 2 𝑥→+∞ √1− 1 √1−0
𝑥 𝑥 𝑥2

THEOREM 2:
+∞, 𝑟 𝑖𝑠 𝑒𝑣𝑒𝑛
a. lim 𝑥 𝑟 = +∞ b. lim 𝑥 𝑟 = {
𝑥→+∞ 𝑥→−∞ −∞, 𝑟 𝑖𝑠 𝑜𝑑𝑑
EXAMPLES:
𝑥4 1 1
𝑥 4 +1 + 𝑥2 + 2 𝑥 2 +0 𝑥2
𝑥2 𝑥2
1. lim = lim 2 3𝑥 = lim 3
𝑥
= lim = lim = lim 𝑥 2 = + ∞
𝑥→−∞ 𝑥 2 +3𝑥 𝑥
𝑥→−∞ + 𝑥→−∞ 1+ 𝑥→−∞ 1+0 𝑥→−∞ 1 𝑥→−∞
𝑥2 𝑥2 𝑥

3 2
2. lim (−6𝑥 5 + 3𝑥 2 − 2) = lim 𝑥 5 (−6 + − )
𝑥→+∞ 𝑥→+∞ 𝑥3 𝑥5
= +∞(−6 + 0 − 0)
= +∞(−6)
= −∞

BASIC CALCULUS

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