All Junior Cert Mathematics posts
• #### Equation of a Line... HELP!!!!!! hamzamughees

There are three different formulae for the equation of a line

y= mx + x

y - y1 = m(x - x1)

ax + by + c = 0

Which one is easier and works all the time.

1. #### Sarah2014

my teacher says you need to learn all three i get so mixed up on these too!!

2. #### hamzamughees

Which one do you think is easiest though?

3. #### Sarah2014

it depends on the question given but in my opinion i find ax+by+c=0 easy

Thanx

5. #### Sarah2014

youre welcome

6. #### Chris241197

i've never learnt ax+by+c =0 and still got A in jc maths, and the fist and second formulae are just variations on each other so you only need to learn the y-y1=m(x-x1), however ironically you don't need to learn any of these as they are in the formula book.

7. #### Sarah2014

but how would you know which one to use then? @chris241197

8. #### 1357

it says in the formula book what you use them for

ok thanks

10. #### hamzamughees

When I learned these formulae my teacher said that each of these equations are better in some occasions.

11. #### Sarah2014

im so confused like how do you know when reading a question which formula you use?

exactly

13. #### Sarah2014

ughhh so confusing haha

14. #### Roberto Mane

It usually tells you in the question,like in an area and volume question where it tells you to express your answer in terms of pi.

15. #### Sarah2014

oh ok thanks

16. #### themathstutor.ie

y= mx + x

should be

y= mx + c

Eamonn - TheMathsTutor.ie

17. #### hamzamughees

Oh yea I noticed that later

18. #### hamzamughees

Sorry my bad and thanx

19. #### bridgetown1

y=mx+c gives you the slope (m, rises/run) and the y intercept.

this equation is easiest to work with, makes graphing easier and can be worked out from a well drawn graph.

20. #### themathstutor.ie

y - y1 = m(x - x1)

is used if you have the slope of the line and the coordinates of a point on the line. If you know these, you can sub them in to find out the equation of the line.

And if you don't know the slope, but have 2 points, then there is a formula to find the slope of the line between those points, so do that first, then use the above formula to find the equation of the line.

Eamonn - TheMathsTutor.ie

21. #### themathstutor.ie

As for

ax + by + c = 0

This form is not as easy to interpret as y = mx + c

You could extract some info about the slope and the y intercept e.g. the slope would be -a/b, and the y intercept is -c/b in this case.

But you might find it easier to rearrange whatever equation you are given, so as to put it in the form y=mx+c. In doing so, you need to be careful to change the sign of any term you bring across the = sign, and also if you divide one term, you must divide them all by the same.

Once in the form y = mx + c, it's easy to interpret.

Eamonn - TheMathsTutor.ie

22. #### hamzamughees

thank you so much guys.

23. #### Me

Share files from your computer