Calculus Cheat Sheet Limits

Calculus Cheat Sheet

Limits Definitions Limit at Infinity : We say lim f ( x ) = L if we Precise Definition : We say lim f ( x ) = L if x ®a

x ®¥

for every e > 0 there is a d > 0 such that whenever 0 < x - a < d then f ( x ) - L < e .

can make f ( x ) as close to L as we want by taking x large enough and positive.

“Working” Definition : We say lim f ( x ) = L

There is a similar definition for lim f ( x ) = L

if we can make f ( x ) as close to L as we want by taking x sufficiently close to a (on either side of a) without letting x = a .

except we require x large and negative.

x ®a

Right hand limit : lim+ f ( x ) = L . This has x ®a

the same definition as the limit except it requires x > a . Left hand limit : lim- f ( x ) = L . This has the x ®a

x ®-¥

Infinite Limit : We say lim f ( x ) = ¥ if we x ®a

can make f ( x ) arbitrarily large (and positive) by taking x sufficiently close to a (on either side of a) without letting x = a . There is a similar definition for lim f ( x ) = -¥ x ®a

except we make f ( x ) arbitrarily large and negative.

same definition as the limit except it requires x
x ®a

x ®a

x ®a

x ®a

x ®a

lim f ( x ) ¹ lim- f ( x ) Þ lim f ( x ) Does Not Exist

x ®a +

x ®a

x ®a

Properties Assume lim f ( x ) and lim g ( x ) both exist and c is any number then, x ®a

x ®a

1. lim éëcf ( x ) ùû = c lim f ( x ) x ®a x ®a 2. lim éë f ( x ) ± g ( x ) ùû = lim f ( x ) ± lim g ( x ) x ®a x®a x ®a 3. lim éë f ( x ) g ( x ) ùû = lim f ( x ) lim g ( x ) x ®a x ®a x ®a

f ( x) é f ( x ) ù lim 4. lim ê = x ®a provided lim g ( x ) ¹ 0 ú x ®a x ®a g ( x ) g ( x) ë û lim x ®a n

n 5. lim éë f ( x ) ùû = élim f ( x ) ù x ®a ë x ®a û 6. lim é n f ( x ) ù = n lim f ( x ) û x ®a ë x®a

Basic Limit Evaluations at ± ¥ Note : sgn ( a ) = 1 if a rel="nofollow"> 0 and sgn ( a ) = -1 if a < 0 . 1. lim e x = ¥ & x®¥

2. lim ln ( x ) = ¥ x ®¥

lim e x = 0

x®- ¥

&

lim ln ( x ) = - ¥

x ®0 -

b =0 xr 4. If r > 0 and x r is real for negative x b then lim r = 0 x ®-¥ x 3. If r > 0 then lim

x ®¥

5. n even : lim x n = ¥ x ®± ¥

6. n odd : lim x n = ¥ & lim x n = -¥ x ®¥

x ®- ¥

7. n even : lim a x + L + b x + c = sgn ( a ) ¥ n

x ®± ¥

8. n odd : lim a x n + L + b x + c = sgn ( a ) ¥ x ®¥

9. n odd : lim a x n + L + c x + d = - sgn ( a ) ¥

Visit http://tutorial.math.lamar.edu for a complete set of Calculus notes.

x ®-¥

© 2005 Paul Dawkins

Calculus Cheat Sheet

Evaluation Techniques Continuous Functions L’Hospital’s Rule f ( x) 0 f ( x) ± ¥ If f ( x ) is continuous at a then lim f ( x ) = f ( a ) x ®a If lim = or lim = then, x ®a g ( x ) x ®a g ( x ) 0 ±¥ Continuous Functions and Composition f ( x) f ¢( x) lim = lim a is a number, ¥ or -¥ f ( x ) is continuous at b and lim g ( x ) = b then x ®a g ( x ) x ®a g ¢ ( x )

(

)

x ®a

lim f ( g ( x ) ) = f lim g ( x ) = f ( b ) x ®a

x ®a

Polynomials at Infinity p ( x ) and q ( x ) are polynomials. To compute

Factor and Cancel ( x - 2 )( x + 6 ) x 2 + 4 x - 12 lim = lim 2 x®2 x®2 x - 2x x ( x - 2)

p ( x) factor largest power of x out of both x ®± ¥ q ( x ) lim

x+6 8 = lim = =4 x®2 x 2 Rationalize Numerator/Denominator 3- x 3- x 3+ x lim 2 = lim 2 x ®9 x - 81 x ®9 x - 81 3 + x 9- x -1 = lim = lim 2 x ®9 ( x - 81) 3 + x x®9 ( x + 9 ) 3 + x

(

)

(

p ( x ) and q ( x ) and then compute limit.

(

(

)

Piecewise Function

)

-1 1 =(18)( 6 ) 108 Combine Rational Expressions 1æ 1 1ö 1 æ x - ( x + h) ö lim ç - ÷ = lim çç ÷ h ®0 h x + h x ø h®0 h è x ( x + h ) ÷ø è 1 æ -h ö 1 -1 = lim çç = lim = ÷ h ®0 h x ( x + h ) ÷ h®0 x ( x + h ) x2 è ø =

)

x 2 3 - 42 3 - 42 3x 2 - 4 3 x lim = lim 2 5 = lim 5 x = ®¥ x ®-¥ 5 x - 2 x 2 x ®-¥ x x 2 x -2 x -2 ì x 2 + 5 if x < -2 lim g ( x ) where g ( x ) = í x ®-2 î1 - 3x if x ³ -2 Compute two one sided limits, lim- g ( x ) = lim- x 2 + 5 = 9 x ®-2

x ®-2

x ®-2+

x ®-2

lim g ( x ) = lim+ 1 - 3 x = 7

One sided limits are different so lim g ( x ) x ®-2

doesn’t exist. If the two one sided limits had been equal then lim g ( x ) would have existed x ®-2

and had the same value.

Some Continuous Functions Partial list of continuous functions and the values of x for which they are continuous. 1. Polynomials for all x. 7. cos ( x ) and sin ( x ) for all x. 2. Rational function, except for x’s that give 8. tan ( x ) and sec ( x ) provided division by zero. 3p p p 3p 3. n x (n odd) for all x. x ¹ L , - , - , , ,L 2 2 2 2 4. n x (n even) for all x ³ 0 . 9. cot ( x ) and csc ( x ) provided 5. e x for all x. x ¹ L , -2p , -p , 0, p , 2p ,L 6. ln x for x > 0 . Intermediate Value Theorem Suppose that f ( x ) is continuous on [a, b] and let M be any number between f ( a ) and f ( b ) . Then there exists a number c such that a < c < b and f ( c ) = M .

Visit http://tutorial.math.lamar.edu for a complete set of Calculus notes.

© 2005 Paul Dawkins