Prove that \frac{df(c)-cf(d)}{f(d)-f(c)}=\frac{f(\xi)}{f'(\xi)}-\xi for some \xi\in(c,d)

deiteresfp

deiteresfp

Answered question

2022-01-14

Prove that
df(c)cf(d)f(d)f(c)=f(ξ)f(ξ)ξ
for some ξ(c,d)

Answer & Explanation

Jonathan Burroughs

Jonathan Burroughs

Beginner2022-01-15Added 37 answers

Step 1
According to Cauchy's version of Mean Value Theorem, there exists an ξ(c,d), such that
cf(d)df(c)f(d)f(c)=cf(c)df(d)1f(c)1f(d)
=(ξf(ξ))(1f(ξ))
=f(ξ)ξf(ξ)f2(ξ)f(ξ)f2(ξ)
=f(ξ)ξf(ξ)f(ξ)
=ξf(ξ)f(ξ)
Beverly Smith

Beverly Smith

Beginner2022-01-16Added 42 answers

Step 1
Let
κ=df(c)cf(d)f(d)f(c)
and see
cd1κ+ξdξ=ln(κ+dκ+c)
=ln((dc)f(d)f(d)f(c)(dc)f(c)f(d)f(c))
=lnf(d)f(c)
Also,
cdf(ξ)f(ξ)dξ=lnf(ξ)cd=lnf(d)f(c)
So, since these are both continuous functions of ξ, with the same integral, they must agree somewhere.

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