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# Complete The Square Question watch

1. Complete the square and please show me the working out as I am confused on how does the answer get the +10 as the rest I understand

5x^2-20x+30
2. (Original post by Daydreamer3)
Complete the square and please show me the working out as I am confused on how does the answer get the +10 as the rest I understand

5x^2-20x+30
It is best to post your working so we can correct it.. otherwise it is just giving you the answer

You must have done almost all of it right anyway, I suspect it is just the part outside the brackets.

3. (Original post by SeanFM)
It is best to post your working so we can correct it.. otherwise it is just giving you the answer

You must have done almost all of it right anyway, I suspect it is just the part outside the brackets.

5(x^2-4)+30
5[(x-2)]^2-4+30
5[(x-2)]^2+16

OR

5[x^2-4x+6)

I dont even know what I did aha.... :/
4. (Original post by Daydreamer3)
5(x^2-4)+30
5[(x-2)]^2-4+30
5[(x-2)]^2+16

OR

5[x^2-4x+6)

I dont even know what I did aha.... :/
Okay, side question that will help (your last line is the correct way)

How do you complete the square for x^2 - 4x + 6?
5. (Original post by SeanFM)
Okay, side question that will help (your last line is the correct way)

How do you complete the square for x^2 - 4x + 6?
(x-2)^2-4+6

(x-2)^2+2
6. (Original post by Daydreamer3)
(x-2)^2-4+6

(x-2)^2+2
Good, so what is 5x^2 - 20x + 30 in completed square form, using the answer you have just given me?
7. (Original post by SeanFM)
Good, so what is 5x^2 - 20x + 30 in completed square form, using the answer you have just given me?
ummmm...

5(x-2)^2-4+6??

Is it supposed to be 4+6 I dont know......!
8. (Original post by Daydreamer3)
ummmm...

5(x-2)^2-4+6??

Is it supposed to be 4+6 I dont know......!
If you are asked to complete the square for 5x^2 - 20x + 30 which you have rewritten as 5(x^2 -4x + 6)

and you know that the completed square of (x^2 - 4x + 6) is (x-2)^2 + 2..

then the completed square of 5(x^2 - 4x + 6) = 5(x-2)^2 + ....
9. (Original post by SeanFM)
If you are asked to complete the square for 5x^2 - 20x + 30 which you have rewritten as 5(x^2 -4x + 6)

and you know that the completed square of (x^2 - 4x + 6) is (x-2)^2 + 2..

then the completed square of 5(x^2 - 4x + 6) = 5(x-2)^2 + ....
Yes I know it is +10 but what I dont understand is where it has come from.
Can you just please tell me the answer we have spent too long working this out ahah
10. (Original post by Daydreamer3)
Yes I know it is +10 but what I dont understand is where it has come from.
Can you just please tell me the answer we have spent too long working this out ahah
It has come from multiplying the whole thing by 5...

I'll pick up with my previous post and then if you are still stuck then ask

(I know there is a slightly different method where you take out the 'a' in ax^2 + bx +c and divide through by it etc etc)

If the completed square of and is 5 times that, then the completed square of that is simply
11. (Original post by SeanFM)
It has come from multiplying the whole thing by 5...

I'll pick up with my previous post and then if you are still stuck then ask

(I know there is a slightly different method where you take out the 'a' in ax^2 + bx +c and divide through by it etc etc)

If the completed square of and is 5 times that, then the completed square of that is simply
Can you please just show the working out directly from 5x^2-20x+30 instead of using the other form as that would be more helpful.
12. (Original post by Daydreamer3)
Can you please just show the working out directly from 5x^2-20x+30 instead of using the other form as that would be more helpful.
5x²-20x+30
5(x²-4x)+30
5[(x-2)²-4]+30
5(x-2)²+30-20
5(x-2)²+10
13. (Original post by Daydreamer3)
Can you please just show the working out directly from 5x^2-20x+30 instead of using the other form as that would be more helpful.
Let me know which bit of what I have shown is confusing.. if not just read it over and see if you get it

I'm not sure what method you're used to as you haven't shown one for an ax term, but you could consider the first two terms and express it as 5(x^2 - 4), which is 5(x-2)^2. and then see that, when expanded, that is equal to 5(x^2 - 4x + 4) which is 5x^2 - 20x + 20 which is 10 less than the original expression, so we'd have to add 10 to the bold expression to give 5(x-2)^2.
14. Here u are, hard to see on pc
Attached Images

15. (Original post by metrize)
5x²-20x+30
5(x²-4x)+30
5[(x-2)²-4]+30
5(x-2)²+30-20
5(x-2)²+10
Oh okayy so you had to multiply the 5 by the 4 I think, thank you!
16. (Original post by SeanFM)
Let me know which bit of what I have shown is confusing.. if not just read it over and see if you get it

I'm not sure what method you're used to as you haven't shown one for an ax term, but you could consider the first two terms and express it as 5(x^2 - 4), which is 5(x-2)^2. and then see that, when expanded, that is equal to 5(x^2 - 4x + 4) which is 5x^2 - 20x + 20 which is 10 less than the original expression, so we'd have to add 10 to the bold expression to give 5(x-2)^2.
Thank you so much for the help!
Here u are, hard to see on pc
Thank youu!!
18. Lmao, the only way you'll truly understand the +10 is if you try and figure it out yourself rather than having someone feed you the information. Better to correct your mistakes yourself

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19. (Original post by Daydreamer3)
Thank youu!!
http://www.mathsgenie.co.uk/gcse.html
20. (Original post by Moltenmo)
Lmao, the only way you'll truly understand the +10 is if you try and figure it out yourself rather than having someone feed you the information. Better to correct your mistakes yourself

Posted from TSR Mobile
Yeah well then I could have just made up anything to get the answer even if wasnt the correct method....

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