is the total noise floor that is made by KTB and Man made noise and phase margin substracte the phase margin?

[TicTek]

Hello guys,
The total Noise floor that is given by the course I am enrolled now is :
Noise Floor = Man_made:noise+KTB-Noise_Margin-----(1)
here I think it is a mistake because for me the noise margin is a margin to have a security margin in case our signal drops below and should be written as:
Noise Floor = Man_made:noise+KTB+Noise_Margin----(2)

do you think the formula (1) is the correct one? is yes, why?

 

[BigBoss]

Noise margin is always positive under all circumstances. Noise quantities are negative ( ought to be ) whatever their sources are.
Noise sources are algebraically added so second one is correct.

 

[TicTek]

Noise margin is always positive under all circumstances. Noise quantities are negative ( ought to be ) whatever their sources are.
Noise sources are algebraically added so second one is correct.

Thank you. I am doing a course on coursera from TU DEFT and the teacher put a minus and I could not get my head around it