Only using 2/3 Motion equations

[Learnphysics]

We all know the three equations of motion

Final speed = initial speed + acceleration *time

change in displacement = initial speed * Time + .5 (acceleration) * (time)^2

FInal velocity ^2 = initial velocity ^2 + (2 * acceleration * displacment)


However i recall reading somewhere (possibly), that you should/can only ever use 2 out of 3 of these equations. Why is this?

[Funkmaster W]

I personally find that i rarely use
FInal velocity ^2 = initial velocity ^2 + (2 * acceleration * displacement) since its only used when you do not have a time and are solving for something else but with the other two you can pretty much solve for anything and even if you think you cant you can still set up simultanious equations with them. the best one of the three is probably change in displacement = initial speed * Time + .5 (acceleration) * (time)^2 since you can use it in just about any situation.

[Phrak]

Really, you only need the definitions of displacement, velocity, and acceleration, and the equations follow as mathematical identities.