I am studying Dummit and Foote Section 15.2. I am trying to understand the proof of Proposition 19 Part (5) on page 682 (see attachment)

Proposition 19 Part (5) reads as follows:

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Proposition 19.

... ...

(5) Suppose M is a maximal ideal and Q is an ideal with for some .

Then Q is a primary idea, with rad Q = M

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The proof of (5) above reads as follows:

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Proof.

Suppose for some where M is a maximal idea.

Then so .

... ... etc

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My problem is as follows:

Why can we be sure that rad M = M?

I know that M is maximal and so no ideal in R can contain M. We also know that

Thus either rad M = M (the conclusion D&F use) or rad M = R?

How do we know that ?

Would appreciate some help.

Peter