@vv 

Your suggestion worked very well!

Thank you so much!

@ecterrab 

As always, perfect!

Thank you very much!

@ecterrab 

You are absolutely right! Thank you very much!

@Torre 

You are completely right! Thanks a lot for your help!

@Kitonum 

Got it! I would like to thank you for your help. I think there is a bug in 2-D math input

See ya!

@Kitonum 

I tried to use your tip but it don't worked.

@Kitonum 

Helped me a lot! Thanks for your help!

@Preben Alsholm 

Sure!

-sqrt(-(2*(-r/(-r+2*M))^eta*M-(-r/(-r+2*M))^eta*r)/r)*X*sqrt(-(-(-r+2*M)/r)^eta*r/(-r+2*M))/f

or

simplification.mw

@vv You are amazing! Thank you very much!

I would like to thank everyone for their support!

@Rouben Rostamian  Sorry! Here is my code:

Download matrix_calculation.mw

I know de matrix U but I can't to find the matrix U

@_Maxim_ 

Sorry, I didn't see your comment! I put here the matrices C and M.

@acer 

Thanks for your help!

I can't apply your suggestion to my problem. The matrices M and C are the following

By the way, I know the matrix U

The point is: I need to reproduce this case to apply in another case.

@acer 

Thanks for your answer!

Unfortunately, the matrix U is a unknown matrix. It isn't numeric matrix.

@Kitonum 

Thanks for your answer!

I use Maple 2017 and still happening,

It changes when I do the command g_[line_element] in package Physics. Later this change causes some problem in the rest calculations.

@ecterrab 

You are amazing! Thanks! I will test!