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# C2 Geometric

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1. In a GP the 2nd term is -12 and the 5th term is 768. Find the common ratio and the first term. how would you do this one?
2. By dividing the terms
3. (Original post by Secreay)
In a GP the 2nd term is -12 and the 5th term is 768. Find the common ratio and the first term. how would you do this one?
We know the formula is

Sub in -12 and n=2 in the formula, then sub in 768 and n=5, you will get two equations, solve them simultaneously.
4. -12/r = 768/r^4 seems wrong :/
5. (Original post by Secreay)
-12/r = 768/r^4 seems wrong :/

You are going right.
6. but i cant cube root i neg number
7. (Original post by Secreay)
but i cant cube root i neg number
why not?
8. (Original post by Secreay)
but i cant cube root i neg number
Negative numbers can be cube rooted but you can't take the square root of them. Input it in your calc, and it will give the answer.
9. HAHA im a noob so r=-4?
10. (Original post by Secreay)
HAHA im a noob so r=-4?
11. (Original post by raheem94)

You are going right.
This is how I went around the equation.

You might find this way easier ^^

-12r^-1=768r^-4
-12=768r-4/r^-1
-12=(768/r^4)/(1/r)
-12=(768/r^4)*(r/1) [doing the dividing fractions so flip the right]
-12=768r/r^4 [now apply indices rules here]
-12=768r^-3
-1/64=r^-3
-1/64=1/r^3
-64/1=r^3/1 [flipped top and bottom]
-4=r
12. (Original post by Stickyelmo)
This is how I went around the equation.

You might find this way easier ^^

-12r^-1=768r^-4
-12=768r-4/r^-1
-12=(768/r^4)/(1/r)
-12=(768/r^4)*(r/1) [doing the dividing fractions so flip the right]
-12=768r/r^4 [now apply indices rules here]
-12=768r^-3
-1/64=r^-3
-1/64=1/r^3
-64/1=r^3/1 [flipped top and bottom]
-4=r
I really don't understand, why you quoted me?

Its not a problem for me to deal with such questions and the way i was doing was easier than your's.

Who's method is better?
13. (Original post by raheem94)
I really don't understand, why you quoted me?

Its not a problem for me to deal with such questions and the way i was doing was easier than your's.

Who's method is better?
his method is better....less booky
14. Both methods are exactly the same.. one looks shorter as a lot of the steps can be done mentally and not written down.
15. (Original post by David_Skiller)
his method is better....less booky
Both methods are exactly the same.. one looks shorter as a lot of the steps can be done mentally and not written down.
Correct, both are the same, but i was surprised to be quoted, may be if the post would have been directed to the OP then it would have made a bit of sense.
16. (Original post by raheem94)
Correct, both are the same, but i was surprised to be quoted, may be if the post would have been directed to the OP then it would have made a bit of sense.

"Who's method is better?"

you were asking the question faget

go back to doing questions in the book
17. ah woops I quoted the wrong text haha X_X

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