Central ideas are the most ideas in a text.

nobody can deny that cars are the most used means of transport all over the world. however owing a car could have some disadvantages. first of all, having a car on your own allows you to go everywhere without depending on timetables. moreover, you can use it whenever you need it no matter the time or the climate conditions. furthermore, if you have a car, you would not have to wait for nobody to share it when going to somewhere. nevertheless, one major disadvantages of owing a car is the fact that fuel is very expensive nowadays so that not everyone can afford it. in addition, you have to pay insurance to be protected by legal conditions. apart from this, the person who owns a car always has to maintain the car and repair any mechanical problem which could costs a huge amount of money. what is more, if you have to travel long distances, and you do not like driving, one probably would be exhausted and irritated due to driving for a long time. besides, cars cause environmental pollution. to sum up, owing a car means that you have more freedom to travel whenever and wherever people want to, however the car maintenance could be a very expensive issue that can have bad consequences at your economy.

. a function is a relation in which each input has only one output. in the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.