The limit evaluates to 1/2.
Steps:
We need to evaluate the limit:
L=limx→0x3tanx−sinx
First, substitute x=0 into the expression:
Numerator: tan(0)−sin(0)=0−0=0
Denominator: 03=0
Since this is an indeterminate form 00, we can use L'Hôpital's Rule or Taylor series expansions.
Method 1: Using L'Hôpital's Rule
Apply L'Hôpital's Rule by taking the derivative of the numerator and the denominator.
1st Application:
Derivative of numerator: dxd(tanx−sinx)=sec2x−cosx
Derivative of denominator: dxd(x3)=3x2
L=limx→03x2sec2x−cosx
Substitute x=0:
Numerator: sec2(0)−cos(0)=12−1=0
Denominator: 3(0)2=0
Still an indeterminate form 00.
2nd Application:
Derivative of numerator: dxd(sec2x−cosx)=2secx(secxtanx)−(−sinx)=2sec2xtanx+sinx
Derivative of denominator: dxd(3x2)=6x
L=limx→06x2sec2xtanx+sinx
Substitute x=0:
Numerator: 2sec2(0)tan(0)+sin(0)=2(1)2(0)+0=0
Denominator: 6(0)=0
Still an indeterminate form 00.
3rd Application:
Derivative of numerator: dxd(2sec2xtanx+sinx)
Using the product rule for 2sec2xtanx:
2[(2secx(secxtanx))tanx+sec2x(sec2x)]+cosx
=2[2sec2xtan2x+sec4x]+cosx
=4sec2xtan2x+2sec4x+cosx
Derivative of denominator: dxd(6x)=6
L=limx→064sec2xtan2x+2sec4x+cosx
Now, substitute x=0:
Numerator: 4sec2(0)tan2(0)+2sec4(0)+cos(0)
=4(1)2(0)2+2(1)4+1
=0+2+1=3
Denominator: 6
L=63=21
Method 2: Using Taylor Series Expansions
Recall the Maclaurin series expansions for sinx and tanx around x=0:
sinx=x−3!x3+5!x5−⋯=x−6x3+O(x5)
tanx=x+3x3+152x5+⋯=x+3x3+O(x5)
Substitute these into the limit expression:
L=limx→0x3(x+3x3+O(x5))−(x−6x3+O(x5))
L=limx→0x3x+3x3−x+6x3+O(x5)
L=limx→0x3(31+61)x3+O(x5)
L=limx→0x3(62+61)x3+O(x5)
L=limx→0x363x3+O(x5)
L=limx→0x321x3+O(x5)
Divide by x3:
L=limx→0(21+O(x2))
As x→0, O(x2)→0.
L=21
The final answer is 1/2.