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

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.