From a text book:
"The general form of a linear equation in two variables is or, where a,b,c are real numbers such that and x,y are variables.
(we often denote the condition a and b are not both zero by .)"
I don’t understand this last condition.
How can we say that represents the condition that a and b are not both zero.
Let a=0,b=1, then also this condition fulfills.