ii) Use an (ε − δ) argument to show that f : R → R be the function defined by
x2−5x−5 if x≥−1 f(x)= x2+x+1 if x<−1
is continuous at x = −1.
1
Expert's answer
2021-05-04T14:36:40-0400
(1)n→∞lim2n23n2−2n+1=23We must show that∀ϵ>0,∃N(ϵ)∈N,such thatn>N(ϵ)⟹∣an−a0∣<ϵ∣an−a0∣=∣∣2n23n2−2n+1−23∣∣=∣∣2n23n2−2n+1−3n2∣∣=∣∣2n2−2n+1∣∣−2n+1<−2n+2<2n−2<2n∣an−a0∣=∣∣2n2−2n+1∣∣<∣∣2n22n∣∣=∣∣n1∣∣⟹−ϵ<n1<ϵ⟹n>ϵ1∴∃aN(ϵ)∈Nsuch thatn>N(ϵ)⟹∣an−a0∣<ϵwhich verifies the limit.(2)f(x)=x2−5x−5ifx≥−1,f(x)=x2+x+1ifx<−1f(−1)=(−1)2−5(−1)−5=1We must show that if∣x−(−1)∣<δ⟹∣x+1∣<δ,∣f(x)−f(−1)∣<ϵ∣f(x)−f(−1)∣=∣x2−5x−5−1∣<ϵ∣x2−5x−6∣⟹∣x2+2x+1∣⟹∣(x+1)2∣<∣x2+2x+1∣<ϵ<ϵ⟹∣x+1∣<ϵ⟹δ=ϵThis implies that∣f(x)−f(−1)∣<ϵf(−1)=(−1)2+(−1)+1=1We must show that if∣x−(−1)∣<δ⟹∣x+1∣<δ,∣f(x)−f(−1)∣<ϵ∣f(x)−f(−1)∣=∣x2+x−2∣<ϵ∣x2+x−2∣⟹∣x2+2x+1∣⟹∣(x+1)2∣<∣x2+2x+1∣<ϵ<ϵ⟹∣x+1∣<ϵ⟹δ=ϵThis implies that∣f(x)−f(−1)∣<ϵHence,f(x)is continuous atx=1.
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