Is there an equation for flat shading?

To my understanding, it is this:

I = Ia x Ka + Fatt x Il x (Kd x (N x L))

Final intensity = ambient reflection x ambient reflection coefficient + (attenuation x incident light intensity x (diffusive reflection coefficient x (surface normal x Light intensity))

However, I have only found one source for this equation (my lecture slides).

Are there any other sources to confirm this is the equation for flat shading? I can't find one.

Is there an equation for flat shading?

To my understanding, it is this:

$$I = I_a \times K_a + F_{att} \times I_l \times (K_d \times (N \times L))$$

Final intensity = ambient reflection x ambient reflection coefficient + (attenuation x incident light intensity x (diffusive reflection coefficient x (surface normal x Light intensity))

However, I have only found one source for this equation (my lecture slides).

Are there any other sources to confirm this is the equation for flat shading? I can't find one.