# Implementation process of mixed finite element method

**URL:** https://community.freefem.org/t/implementation-process-of-mixed-finite-element-method/1866
**Category:** General Discussion
**Created:** [July 2, 2022, 8:08am UTC](https://community.freefem.org/t/implementation-process-of-mixed-finite-element-method/1866 "2022-07-02T08:08:40Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![Qianrui](https://avatars.discourse-cdn.com/v4/letter/q/9fc348/32.png) [@Qianrui](https://community.freefem.org/u/Qianrui)
#### Post date: [July 2, 2022, 8:08am UTC](https://community.freefem.org/t/implementation-process-of-mixed-finite-element-method/1866/1 "2022-07-02T08:08:40Z")

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Hello, I am using mixed finite element method to solve a PDE. Theoretically，

the following two equations are obtained, in which there are two unknown quantities u, p,

**A \* u+B \* p=b1;**

**C \* p+D \* u=b2;**

How does freefem++ implement this solution process?

Is p=C^ (-1) \* (b2-D\*u) brought into the first equation group? If matrix C is irreversible?