# How to show that if $A, B$, and $A + B$ are invertible matrices with the same size, then $A(A^{ −1} + B^{ −1} )B(A + B)^ {−1} = I$ [duplicate]

This question already has an answer here:

1. Show that if $A$, $B$, and $A + B$ are invertible matrices with the same size, then $$A(A^{-1} + B^{-1})B(A + B)^{-1} = I$$
2. What does the result in part $1$ tell you about the matrix $A^{-1} + B^{-1}$?

Ok I never came across any identities that would allow me to cancel these values out in the book so far... So I have no idea how I'm supposed to "show" that they are equal... So how do I solve/show this?

Note: for those who say this is a duplicate, read both questions again, this question asks how to solve the first part, the other question asks and only answers how to solve the second part. the first part isn,t explained in the other question, only the second part.

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## marked as duplicate by Algebraic Pavel, Praphulla Koushik, amWhy, user88595, Davide GiraudoJun 16 '14 at 13:28

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

yeah.. but it doesnt really answer.. –  J L Jun 16 '14 at 12:55
@JL The answer is right there. –  M. Vinay Jun 16 '14 at 12:55
it answers the second part but not the first part –  J L Jun 16 '14 at 12:56
How come? The identity tells you that $(A^{-1}+B^{-1})B(A+B)^{-1}A=I$ (multiply by $A^{-1}$ from left and then by $A$ from right). –  Algebraic Pavel Jun 16 '14 at 12:56
Oh, ok: $A(A^{-1}+B^{-1})B=A+B$, does it help? –  Algebraic Pavel Jun 16 '14 at 12:57

## 1 Answer

$$A(A^{-1}+B^{-1})B(A+B)^{-1} = I \iff (A^{-1}+B^{-1}) = A^{-1}B^{-1}(A+B) \\\iff A^{-1}+B^{-1} = B^{-1}+A^{-1}$$

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