Hi all, Say i have 2 equations, s =t and 1-s = t-1. Normally one would solve the equations simply but subbing in S=t into the other equation and obtain t = s = 1. However, if i were to slightly rearrange this equation: 1-s = t-1 I can obtain this : -1(s-1) = (t-1) -1 = (t-1)/(s-1), wouldn't this not make sense if i sub s=t into it? And i am anticipating that some would point out that by rearranging the equation i divided by 0. So the whole thing is not valid. However what if i rewrote my whole post and state first that initially i have 2 equations, s=t and 1 = (t-1)/(1-s) and i wish to solve this. Then i would have solve it like this : 1 = (t-1)/(1-t) 1-t = t-1 t=1 = s However if this is the case then wouldn't the original equation 1 = (t-1)/(1-s) is self is invalid? What is wrong here? or did i go something wrongly? By the way, i came across this thing as i am doing my vectors -Planes tutorial. Any help will be greatly appreciated.