In my last doubt, I asked to give a formal proof, for (-)(+)=(-). I was asked by my maths teacher in school, that how when 2 unlike signs are multiplied, why the result is a negative value. Neha ma'am answered this last time, but it was not at all satisfactory.. I wanted one of the formal proofs like Brijendra Pal sir gave me for (-)(-)=(+)..Experts may prefer to that one, to make the answer satisfactory for (-)(+)=(-)...

Dear Student,

From the distributive law of multiplication we havea×b+c=a×b+a×cwhich holds for all real numbers.So it will hold if we put a=1, b=-1 and c=1 1×-1+1=1×-1+1×1since -1+1=0 we get1×0=1×-1+10=1×-1+1after changing sides1×-1=-1

Hence proved 

Regards,

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I hope to get a very satisfactory answer this time,like Brijendra sir gave me for (-)(-)=(+) .. Even if experts take more time to answer this, then too I want a very specific answer. But please don't the answer, like, when 2 unlike signs are multiplied ,we get a negative value... Hope you are very aware of my doubt , very accurately. .
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I want a very proper explained proof for this... Even it takes more time, please answer specifically, according to the proper agenda mentioned in the question...
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Hoping to get a very better answer this time.. Please...
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