I've always thought you cannot add up denominators

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The discussion revolves around the mathematical principle that you cannot simply add fractions by summing their numerators and denominators unless certain conditions are met. A user presents a scenario involving sine ratios and demonstrates algebraic manipulation that leads to a conclusion about the equality of ratios. While initially confused, they realize that the geometric relationships involved allow for the preservation of the ratio when adding equal fractions. The conversation highlights the importance of understanding the conditions under which such operations are valid. Ultimately, the participants confirm that the algebraic approach is correct under specific circumstances.
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Is this right?
I've always thought you cannot add up denominators...
 
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Let a/sin55=b/sin80=c/sin45=D
then a=Dsin55, b=Dsin80, c=Dsin45
(a+b+c)/(sin55+sin80+sin45)
=(Dsin55+Dsin80+Dsin45)/(sin55+sin80+sin45)
=D(sin55+sin80+sin45)/(sin55+sin80+sin45)
=D
 
Welcome to Physics Forums, nevmx.

That sum is not correct.

Is that all you need, or are you looking for a reason?
 
Some Pig said:
Let a/sin55=b/sin80=c/sin45=D
then a=Dsin55, b=Dsin80, c=Dsin45
(a+b+c)/(sin55+sin80+sin45)
=(Dsin55+Dsin80+Dsin45)/(sin55+sin80+sin45)
=D(sin55+sin80+sin45)/(sin55+sin80+sin45)
=D

I think I stand corrected: the algebra is compelling, but I am not seeing what is going on. It looks like some geometric relationship is setting a constraint that makes it possible.

The denominator is 2.511. I think what's confusing me is that there are an number of values for a, b and c given those angles...
 
nevmx, it is wrong to replace some fractions added together with a signal fraction by adding the the numerators together and dividing that by the sum of the denominators.

If some fractions are equal to each other that is a different situation, though. Apparently, these fractions are also all equal to the sum of the numerators divided by the sum of the denominators. This can be shown using a technique like Some Pig showed.
 
I think I see why now. This is saying that when you add three equal ratios together, the ratio is preserved in the sum.
1/2 + 2/4 + 5/10 = 8/16

Pretty cool.
 
welcome to pf!

hi nevmx! welcome to pf! :smile:
nevmx said:
Is this right?

short answer: yes :biggrin:

longer answer: if the ratio is 1/r, it's (a + b + c)/(ra + rb + rc) :wink:
 
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