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# urgent coordinate geometry Help please !! watch

1. Hi guys i need urgent help. We have only done 1 lesson on coordinate geometry and i don't understand my task that well. Ive done most of my questions already but there are some i dont know where to begin with. Can someone please guide me through what i should do in the following question?? Any help will be really appreciated and thanks in advance to anyone who does

question:
1) the coordinates of two points are A(-1,-3) and B(5,7).

i) find the equation of the line l perpendicular to AB and passes through the point B.

ii) the lines l crosses the y axis at point C. Find the area of triangle ABC, simplifying answer.

So for part 1 do i start by finding the gradient of AB? I got that to be 5/3 so the perpendicular line will have a gradient of -3/5 ?
Since it passes through B would i do
y= mx+c
Where m= -3/5 y= 7 and x= 5
So c= 10 ??
2. (Original post by Aty100)
Hi guys i need urgent help. We have only done 1 lesson on coordinate geometry and i don't understand my task that well. Ive done most of my questions already but there are some i dont know where to begin with. Can someone please guide me through what i should do in the following question?? Any help will be really appreciated and thanks in advance to anyone who does

question:
1) the coordinates of two points are A(-1,-3) and B(5,7).

i) find the equation of the line l perpendicular to AB and passes through the point B.

ii) the lines l crosses the y axis at point C. Find the area of triangle ABC, simplifying answer.

So for part 1 do i start by finding the gradient of AB? I got that to be 5/3 so the perpendicular line will have a gradient of -3/5 ?
Since it passes through B would i do
y= mx+c
Where m= -3/5 y= 7 and x= 5
So c= 10 ??
The first part is correct.
For the second part, have you found the coordinates of C? Once you've done that, plot the points on a set of axes to see what this triangle looks like and deduce how you might go about finding its area.
3. (Original post by razzor)
The first part is correct.
For the second part, have you found the coordinates of C? Once you've done that, plot the points on a set of axes to see what this triangle looks like and deduce how you might go about finding its area.
Sorry how would i find the coordinates of C ??
Ive got the equation of the first part as
Y= -3/5x + 10
Would the coordinates therefore be 3/5 and -10 ??
4. (Original post by Aty100)
Sorry how would i find the coordinates of C ??
Ive got the equation of the first part as
Y= -3/5x + 10
Would the coordinates therefore be 3/5 and -10 ??
It says the graph crosses the y axis at C. When a graph crosses the y axis, x=0. You can use this to find the y coordinate.
5. (Original post by Aty100)
Sorry how would i find the coordinates of C ??
Ive got the equation of the first part as
Y= -3/5x + 10
Would the coordinates therefore be 3/5 and -10 ??
We know that this line crosses the y-axis at C, so what x-coordinate does it have?
6. (Original post by razzor)
We know that this line crosses the y-axis at C, so what x-coordinate does it have?
Is C (0,10) ??

Also in the attachment is my attempt at the area,
I'm not really sure what to do, do we use the formula to find the area of a triangle?
Attached Images

7. (Original post by Aty100)
Is C (0,10) ??

Also in the attachment is my attempt at the area,
I'm not really sure what to do, do we use the formula to find the area of a triangle?
As to your second question: yes, use 1/2 * base * height
8. (Original post by Zacken)
As to your second question: yes, use 1/2 * base * height
But which base x height do I use ??

So I do

1/2* 4 * 3 ??
= 6 ?

Or

1/2 * 4 * 13
=26 ?
Im lost
9. (Original post by Aty100)
But which base x height do I use ??

So I do

1/2* 4 * 3 ??
= 6 ?

Or

1/2 * 4 * 13
=26 ?
Im lost
Where did the 4 and 13 come from?
You need to calculate the distance between the coordinates and then apply the area formula.
Spoiler:
Show
Distance between two set of coordinates (x1, x2) and (y1, y2) is given by
10. (Original post by razzor)
Where did the 4 and 13 come from?
You need to calculate the distance between the coordinates and then apply the area formula.
Spoiler:
Show
Distance between two set of coordinates (x1, x2) and (y1, y2) is given by
Thanks so much,
I got confused
So i find the distance between A & C?
11. (Original post by Aty100)
Thanks so much,
I got confused
Spoiler:
Show
So i find the distance between A & C?

Not quite - the distance AC isn't the height of the triangle.
Have a look where the right-angle is formed (identify which line is l)
12. (Original post by Aty100)
Is C (0,10) ??

Also in the attachment is my attempt at the area,
I'm not really sure what to do, do we use the formula to find the area of a triangle?
Or you could do this:

Working out the area of right-angled triangles is probably easier than working out the equation perpendicular line for the height and finding out where it meets the base line and how long it is.

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