Q. If f(x)=(x2+1)3, what is limx→−1x+1f(x)−f(−1)?
Substitute x=−1: First, substitute x=−1 into f(x) to find f(−1).f(−1)=((−1)2+1)3=(1+1)3=23=8.
Simplify expression: Now, simplify the expression (f(x)−f(−1))/(x+1).(f(x)−f(−1))/(x+1)=((x2+1)3−8)/(x+1).
Factorize the numerator: To find the limit as x approaches −1, we need to simplify the numerator. Notice that x2+1=2 when x=−1, so we can factorize the numerator using the difference of cubes: (x2+1)3−8=(2)3−8=8−8=0. This shows that the numerator is zero at x=−1.
Check denominator: Since the numerator is zero at x=−1, we need to check if the denominator also becomes zero to avoid division by zero.At x=−1, the denominator x+1=−1+1=0.
Apply L'Hopital's Rule: Both the numerator and denominator are zero at x=−1, indicating a 0/0 form. We apply L'Hopital's Rule:Differentiate the numerator and the denominator separately.Numerator: dxd[(x2+1)3]=3(x2+1)2⋅2x=6x(x2+1)2.Denominator: dxd[x+1]=1.
Find limit of derivatives: Now, find the limit of the derivatives as x approaches −1: limx→−1[16x(x2+1)2]=6(−1)(1+1)2=6(−1)(2)2=−24.
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