Graphing log vs X

[MrNeuro]

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How is it A please explain how does

y=klogx give you A???

 

[milski]

It's plotting x vs logM, not x vs M. The relation between x and log M is linear, if the formula that you have written is correct. I'm not sure what the graph is supposed to represent but if log M is expected to decrease with distance, A would be the correct graph.

 

[MrNeuro]

It's plotting x vs logM, not x vs M. The relation between x and log M is linear, if the formula that you have written is correct. I'm not sure what the graph is supposed to represent but if log M is expected to decrease with distance, A would be the correct graph.

ahhhhh soo your have to plug in a value for D to get log M not for log M to get D????

why would log M decrease with distance if D increases logM should increase.....where am i wrong here???

im doing log 10 = 1 D = 1 plot 1,1

log 100 = 2 D=2 plot 2,2
log 1000 = 3 D=3 plot 3,3

its a positive slope??? why is it negative?

and why would you expect log M to decrease with distance

 

[ljc]

ahhhhh soo your have to plug in a value for D to get log M not for log M to get D????

why would log M decrease with distance if D increases logM should increase.....where am i wrong here???

im doing log 10 = 1 D = 1 plot 1,1

log 100 = 2 D=2 plot 2,2
log 1000 = 3 D=3 plot 3,3

its a positive slope??? why is it negative?

and why would you expect log M to decrease with distance

A general rule of thumb for MCAT logs that seems to work for me: I've never seen a log question where the answer WASN'T a straight slope. And since this is "-log(x)", you know it has a negative slope. So pick the straight line with the negative slope.

 

[MrNeuro]

A general rule of thumb for MCAT logs that seems to work for me: I've never seen a log question where the answer WASN'T a straight slope. And since this is "-log(x)", you know it has a negative slope. So pick the straight line with the negative slope.

it says a-b log M

where A and B are some constant....how are you supposed to know that its negative??

 

[SaintJude]

Actually, yes, I have noticed that usually the answers regarding log graphs are straight slopes, but I don't know why. For the purposes of the MCAT, semi-log plots convert exponential curves into straight lines. Was there an original graph that plotted an exponential curve?

Beyond the straightness of the line, the negative slop made sense to me:

Just don't get hung up on log

D= a-b(log M)
D-a= - b(log M) (multiply by -1)

a-D = b (log M )

a-D/ b = log M

As you increase D, it should be clear that the overall fraction will become smaller and smaller. So log M is definitely decreasing.

 

[ljc]

it says a-b log M

where A and B are some constant....how are you supposed to know that its negative??

Y=mx+b
X="logM"
b="a"
m="-b"

Y=(-b)(logM)+a

(-b) is the slope. The slope is negative.

 

[SaintJude]

That's a much more valid solution than I offered. Thanks ljc.

 

[MrNeuro]

Y=mx+b
X="logM"
b="a"
m="-b"

Y=(-b)(logM)+a

(-b) is the slope. The slope is negative.

this stuff usually isn't that hard for me but

shouldn't "logM" be Y and X be D ????


thanks SaintJude for the explanation i got it using your way just trying to understand ljc's