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# need a bit of an explanation on logs watch

1. so
5^n-3 = 1/25

i know i have to change the base to 5. thats easy because you dont have to use a calc.
5^n-3 = 5^-2

but...when i get something like 3^2x-1 = 11 i'd have to use the calc and i know the base on calc is 10, but i dont understand the reason why i have to log3 and log11 and do log11/log3? what log law does that follow cos ive covered logs and havent really grasped where that has come from.

but nevertheless without understanding the context, i can just learn the steps and do same type of questions no prob, but im curious as to why i have to do what i have to do to solve it.
2. You have so taking logs of both sides ('any' base) and so we have using log laws (power law).
Notice if you took logs and used the base as 3 or 11 then either the right hand side or left hand side cancels more nicely. You don't even have to specify the base of the log because it doesn't matter the answer will still be the same.
3. Equation isn't clear. Is the 3 in the index or seperate?
4. one first thing i thought of which is so obvious: when solving equations, i do the same on both sides, so i log one side then log the other side... am i rite??

but what does log11 or log 3 actually mean? like shouldnt log11 = to something? why is it standing by itself?
5. (Original post by ihatePE)
so
5^n-3 = 1/25

i know i have to change the base to 5. thats easy because you dont have to use a calc.
5^n-3 = 5^-2

but...when i get something like 3^2x-1 = 11 i'd have to use the calc and i know the base on calc is 10, but i dont understand the reason why i have to log3 and log11 and do log11/log3? what log law does that follow cos ive covered logs and havent really grasped where that has come from.

but nevertheless without understanding the context, i can just learn the steps and do same type of questions no prob, but im curious as to why i have to do what i have to do to solve it.
Power rule for logs. Have you learnt that?
6. (Original post by ihatePE)
one first thing i thought of which is so obvious: when solving equations, i do the same on both sides, so i log one side then log the other side... am i rite??

but what does log11 or log 3 actually mean? like shouldnt log11 = to something? why is it standing by itself?
is the number that satisfies . That's literally its definition here.
7. (Original post by ihatePE)
so
5^n-3 = 1/25

i know i have to change the base to 5. thats easy because you dont have to use a calc.
5^n-3 = 5^-2

but...when i get something like 3^2x-1 = 11 i'd have to use the calc and i know the base on calc is 10, but i dont understand the reason why i have to log3 and log11 and do log11/log3? what log law does that follow cos ive covered logs and havent really grasped where that has come from.

but nevertheless without understanding the context, i can just learn the steps and do same type of questions no prob, but im curious as to why i have to do what i have to do to solve it.
Ok your notation is ambiguous so I am guessing what you intended but here goes ...

Take logarithms of both sides. It doesn't matter which base you use but logs to base 10 are usually introduced first. Later on you will almost always use natural logarithms.

Now a law of logs is so we use this.

Divide both sides by then add 1 and divide by 2.
8. Think of it as the the power you need to raise the smaller number for it to be equal to the larger number.
9. (Original post by NotNotBatman)
Power rule for logs. Have you learnt that?
yes i did, so how did the calculator manage to calculate log11/log3?
is there a nice example that can be done in the head to show how, or its so complicated that only calc can solve it?

(Original post by B_9710)
You have so taking logs of both sides ('any' base) and so we have using log laws (power law).
Notice if you took logs and used the base as 3 or 11 then either the right hand side or left hand side cancels more nicely. You don't even have to specify the base of the log because it doesn't matter the answer will still be the same.
can u show me what it looks like if i use base 3 or 11?
10. (Original post by ihatePE)
yes i did, so how did the calculator manage to calculate log11/log3?
is there a nice example that can be done in the head to show how, or its so complicated that only calc can solve it?

can u show me what it looks like if i use base 3 or 11?
Well what is ? It is what we have to raise 10 to get 11. It is the x in and it's the same with the other log. The example you gave in the op with base 5 is an example for it, but easier as the number is a nice integer.
11. (Original post by ihatePE)
yes i did, so how did the calculator manage to calculate log11/log3?
It used something called the CORDIC algorithm or a variant of it. It isn't something you are expected to be able to work out mentally. Even in the olden days before the invention of electronic calculators you'd have needed to look these numbers up in a booklet. You wouldn't know the value of sin (71) without reaching for your calculator either. It's the same difference.
12. (Original post by NotNotBatman)
Well what is ? It is what we have to raise 10 to get 11. It is the x in and it's the same with the other log. The example you gave in the op with base 5 is an example for it, but easier as the number is a nice integer.
thanks,

so the example ive given first 5^n-3=1/25 will work if i log it as well?
13. (Original post by Mr M)
It used something called the CORDIC algorithm or a variant of it. It isn't something you are expected to be able to work out mentally. Even in the olden days before the invention of electronic calculators you'd have needed to look these numbers up in a booklet. You wouldn't know the value of sin (71) without reaching for your calculator either. It's the same difference.
ah ok, i guess thats the end of it then, dont have to worry about not knowing more
14. (Original post by ihatePE)
thanks,

so the example ive given first 5^n-3=1/25 will work if i log it as well?
What do you mean by it will work? You can find n by taking logs to both sides, because both sides are positive and you "can't" take the log of a negative number.
15. (Original post by ihatePE)
so the example ive given first 5^(n-3)=1/25 will work if i log it as well?
Yes - try it.
16. (Original post by NotNotBatman)
What do you mean by it will work? You can find n by taking logs to both sides, because both sides are positive and you "can't" take the log of a negative number.
dumb worded question, what i meant was i did that question mentally with the knowledge of indices i can change the base. what i couldnt think of was that what i was doing mentally was the same as logs...if you get me, if u dont, dw, i've cracked my confusion now thanks
17. (Original post by Mr M)
Yes - try it.
it comes out the same answer as when i did it mentally
18. (Original post by ihatePE)
thanks,

so the example ive given first 5^n-3=1/25 will work if i log it as well?
log100 is the power we have to raise 10 to to get 100.

10^2 = 100 so log 100 = 2

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