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Calculate without using calculator: log 25 + log 40

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seedrasoso | Student, Grade 10 | eNoter

Posted July 14, 2010 at 7:51 AM via web

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Calculate without using calculator: log 25 + log 40

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hala718 | High School Teacher | (Level 1) Educator Emeritus

Posted July 14, 2010 at 7:53 AM (Answer #1)

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log 25 + log 40

Using the properties with know that:

log x + log y = log x*y

==> log 25 + log 40 = log 25*40

                                 = log 1000

                                 = log 10^3

                                 = 3 log 10

                                  = 3*1 =3

The answer is 3.

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brianevery | High School Teacher | (Level 2) Adjunct Educator

Posted July 14, 2010 at 9:54 AM (Answer #2)

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Calculate without using a calculator log 25 + log 40

On of the rules of logarithms says that the sum of two logs is equal to the log of the product of the two numbers - therefore

log 25+log 40 = log (25*40) = log 1000

Since these logrithms are base 10 we want to write 1000 as 10 raised to some number and the number it is raised to is the answer.

since 10*10=100 and 100*10=1000 we know 1000=10^3

This means that log 1000 = 3 and therefore that log 25+log 40=3

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sandesh007 | Student , Grade 9 | eNoter

Posted July 15, 2010 at 1:40 AM (Answer #3)

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log 40 + log 25

=log (25X40)

=log 1000

=3

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