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leslietaylor leslietaylor
wrote...
11 years ago
So the question on the test was to solve for "y" in the logarithmic equation.

And the most common answers from the people who took the test was: y=1 and/or y=4

So which one was it?
The question (as a picture) is in the following pictoral link:
http://img44.imageshack.us/img44/5020/logproblem.jpg

Please solve this with an explanation! I'm losing it!
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wrote...
11 years ago
log2 y + (1/2)log2 y^6 = 8
log2 y + log2 y^(6/2) = 8
log2 y + log2 y^3 = 8
log2 (y * y^3) = 8
log2 y^4 = 8
4 log2 y = 8
log2 y = 8/4
log2 y = 2
y = 2^2 = 4
Ans: 4
_________________.
microbio24 Author
wrote...
11 years ago
The correct answer is y=4.

Why: You can simplify this to:
log(base2) of y + log(base2) of (y^(6))^(1/2) = log(base 2) 2^8 (you can raise up the 1/2 into the y^6 part of the log also rewrote right side)
log(base 2) y + log(base2) of y^3 = log (base 2) 2^8 (simplifying sqrt(y^6)=y^3)
log(base 2) y^4 = log(base 2) 2^8
y^4=2^8
y=2^2=4

You can plug in y=4 and see that it in fact works. Plugging in the incorrect solution of y=1 yields: 0+0=8, which is obviously incorrect.

Thus, the answer is y=4

[Note: I sort of lied, sqrt(y^6)= absolute value of y^3, but it wasn't relevant here and I didn't want to add extra confusion. This is just a disclaimer and if it confused you ignore it!}
wrote...
11 years ago
Method: collect all possible roots and discard those that don't satisfy the equation (this is a good example of why math teachers shouldn't profess that log(x²) = 2 log(x) is an identity without discussing its limitations; for example, consider x = -1):
log2(y) + 1/2 log2(y^6) = 8
log2(y^8) = 16
y^8 = 65536
y = 4 e^(ki?/4) {k = 0,1,2,3,4,5,6,7}
discard k = 2, 3, 4, 5  (they do not satisfy the equation).
or in standard form,
y = 4, ?2(2+2i), -4i, ?2(2-2i),

Answer: y = 4, 2?2(1 + i), -4i, 2?2(1 - i)
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