IIT JEE , AIEEE Forum - Chemistry , Physical Chemistry - HOw can i take the value of log without see the log tabels for comptition. exams

sukul

HOw can i take the value of log without see the log tabels for comptition. exams

pannaguma

simplify as much as possible, then see whether it lies b/n 1-10 or 10-100, etc
so you will get first digit. now see how many part out of hundred it has coverved from 10^x to 10^x+1.
if it say eg 50%, second digit will usually be 1 greater than 5. ie 6.

bvsatyaram

U are expected to at least remember logarithms of 2 and 3.
Log 10 is 1.
Express the number in terms of 2,3 and 10. (Most of the cases this works.)
Suppose we need to find log 75,
then we do as follows:
75=3*25=3*10*10/(2*2)
So,
log 75 = log3 + 2log10 - 2log2.

Another case could be approximating log.
Consider an MCQ with any pair of its options differ by atleast 1.
Say, the options are a) 5.62 b) 3.2 c) 7.9 d) 21.4
How approximate the nu,ber to nearest power of 10. Suppose, you need to find log (621453), 621453 is between 100000 and 1000000.
So, log (621453) must lie bet 5 and 6.
So, option A.

If neither of the above 2 cases is true, then definitely, logarithms will be given. If not, beleive me, the question will be deleted for evaluation.

chimanshu_007

well nowdays u arent given log tables

but u r given the values in the qs itself (i.e if log 5 will be used then its value will be given)

so dun worry

rooney

Always remember the values of logs of no. from 1 to 10 on your finger tips (base 10 only). Always helps.

log 1=0
log 2=0.301
log 3=0.477
log 4= 0.602=2log 2
log 5=0.645
log 6 = 0.778 = log 3 + log 2
log 7 = 0.845
log 8=3log2 = 0.903
log 9 = 2log3 = 0.954
log 10 = 1

pardesi

i think i wrote this somwhere here just use the expansion of log(1+x) |x| <1

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