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linear graphs question

the picture is a kite with A being the point (2,9) and C which is (8,1). the first question asked to find x which is (5,5), the second part is asking to find the equation of the diagonal DB, how do I do this? please helpImageUploadedByStudent Room1427222797.066397.jpg


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Original post by bubblegumcat
the picture is a kite with A being the point (2,9) and C which is (8,1). the first question asked to find x which is (5,5), the second part is asking to find the equation of the diagonal DB, how do I do this? please helpImageUploadedByStudent Room1427222797.066397.jpg


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did you know that the diagonals of cross at right angles and does that help?
Original post by maggiehodgson
did you know that the diagonals of cross at right angles and does that help?


I do, do I use Pythagoras for that?


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Original post by bubblegumcat
I do, do I use Pythagoras for that?


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Can you find the equation of a straight line given two points? And do you know about the gradients of perpendicular lines?
If you know points D and B you can use y-b=m(x-a) to work out equation
m = gradient (can be worked out from two points)
A = x coordinate of one of the points
B = y coordinate if the same point


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Original post by maggiehodgson
Can you find the equation of a straight line given two points? And do you know about the gradients of perpendicular lines?

do i find the gradient of AC then use that to find the equation of DB, is DB perpendicular to AC?
Original post by It's_Ailie
If you know points D and B you can use y-b=m(x-a) to work out equation
m = gradient (can be worked out from two points)
A = x coordinate of one of the points
B = y coordinate if the same point


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i would if i knew the point of d and b but they don't give it haha, thanks though
Original post by bubblegumcat
i would if i knew the point of d and b but they don't give it haha, thanks though


Ah ah BUT you know that the diagonals cross at a point for which you have calculated the co-ordinates. And I suspect you can now use the gradient and the x and y values to find the equation of the line.

Yes? No?
Original post by maggiehodgson
Ah ah BUT you know that the diagonals cross at a point for which you have calculated the co-ordinates. And I suspect you can now use the gradient and the x and y values to find the equation of the line.

Yes? No?

yes? was i write about the other thing, using the gradient of AC to find the equation of perpendicular DB line?
Original post by bubblegumcat
yes? was i write about the other thing, using the gradient of AC to find the equation of perpendicular DB line?


Yes. The product of the gradients of perpendicular lines is -1 (just in case you needed a reminder)
Original post by maggiehodgson
Yes. The product of the gradients of perpendicular lines is -1 (just in case you needed a reminder)

thanks, i worked out the answer :smile:
Original post by bubblegumcat
thanks, i worked out the answer :smile:



Hurray. Well done.

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