Math Is Fun Forum

Faria

Member

Registered: 2014-12-11

Posts: 8

logarithm

Can anyone solve these maths?
1.if log(x+y) =log x +log y, thn prove, 1/x + 1/y =1
2. If X^2 +y^2=7xy, then prove log(x+y)=log3+ 1/2log xy

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: logarithm

hi Faria

Q1.  Multiplying two numbers is equivalent to adding the logs so simplify log(x) + log(y) to log(xy)

So now you have log(one thing) = log(another) from which you can conclude that (one thing) = another.  That can be changed to the required result by dividing by xy.

Q2. This uses the other log rule that log(A^n) = nlog(A).

Add 2xy to both sides to make a 'perfect' square:

Then divide by 2.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Faria

Member

Registered: 2014-12-11

Posts: 8

Re: logarithm

Ok thank you!