Math CentralQuandaries & Queries


Question from MARICELA, a student:

Need help of how to work this problems

y=2x+1 and y=2x-1

First of all what is the difference?

Second I think is easy just to work with the X y with the line in the middle

but how do you get the numbers for each side.

I want to do my ged, for many years , I am 31 , but I want to learn even though I don't pass the
test this time, I want to know how to resolve the problem
step by step.
Please help me!


You haven't told us what you are to do with these expressions but I expect you are to graph them. You did ask "what is the difference?" and I think that will become clear when you graph them but before you do I can show you one difference immediately.

Each of these functions describes a relationship between two variables x and y. I sometimes think of functions as machines where I input some number x and the machine outputs a number y according to some rule. So for example with the function y = 2x + 1 the machine multiplies the input x by 2 and then adds 1 to the result. Thus if I input x = 0 then the output y is 2 times zero + 1 which is 1. In a similar way if I input x = 0 into the second function y = 2x - 1 the output is y = -1. So her is one difference between the functions,

for y = 2x + 1 if x = 0 then y = +1
for y = 2x - 1 if x = 0 then y = -1

When you say "work with the X y with the line in the middle" i think you are looking at a table of values similar to what I showed Geneva in my response to her question. To generate a similar table for y = 2x + 1 you can select whatever values for x you wish and then calculate the corresponding y value using the expression y = 2x + 1 as I did above with x = 0. I choose x to have small integer values to make the arithmetic easy. Try it with your two functions and write back if you still have difficulties graphing them.


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