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Maths 9-1 question

This is the question
20170211_162045.jpg
And this is my working out so far
I presume you have to solve for a but when I do this I get a cubic equation which can't be right
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14868302739712095218328.jpg507.7KB
sorry I cant seem to get rid of the extra picture, it comes up blank in the edit section
Original post by CraigBackner
This is the question
20170211_162045.jpg
And this is my working out so far
I presume you have to solve for a but when I do this I get a cubic equation which can't be right


Your expression for V is correct!!!

Now you also need to find the value of a a .

Can you make an equation in a a using the fact that the total surface area is 810π 810 \pi ?
(edited 7 years ago)
Did you mean to attach one of your calculation for the surface area? That should give you a, which you can then substitute into your equation for V.

To be dimensionally correct, V must be the cube of length - your posted working looks fine to me.
Original post by crashMATHS
Your expression for V is correct!!!

Now you also need to find the value of a a .

Can you make an equation in a a using the fact that the total surface area is 810π 810 \pi ?


Original post by RogerOxon
Did you mean to attach one of your calculation for the surface area? That should give you a, which you can then substitute into your equation for V.

To be dimensionally correct, V must be the cube of length - your posted working looks fine to me.


I tried formulating an equation involving 810pi and the surface area but when I simplify and try to solve its wrong.
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Attachment not found
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Original post by CraigBackner
I tried formulating an equation involving 810pi and the surface area but when I simplify and try to solve its wrong.


Is (12a)2+(5a)2=17a2 \sqrt{(12a)^2 + (5a)^2} = \sqrt{17a^2} ?
Original post by crashMATHS
Is (12a)2+(5a)2=17a2 \sqrt{(12a)^2 + (5a)^2} = \sqrt{17a^2} ?


ohh I see where I went wrong! instead of squaring the radius and height to work out the slanted height of the cone I only square a
Original post by CraigBackner
ohh I see where I went wrong! instead of squaring the radius and height to work out the slanted height of the cone I only square a


Yeah, a very common mistake to avoid!
Original post by crashMATHS
Yeah, a very common mistake to avoid!


Thanks for the help!!

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