Question 190319
Please help me by answering this question: why minus and minus makes plus?
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Proof:
Let a and b be any real numbers.
Let x=ab+(-a)(b)+(-a)(-b)
.
x= ab+(-a)[(b)+(-b)] factor out -a
=ab+(-a)(0)
=ab+0
=ab
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x=[a+(-a)]b+(-a)(-b)  factor out b
=0*b+(-a)(-b)
=0+(-a)(-b)
=(-a)(-b)
.
x=ab=(-a)(-b)
So, (-a)(-b)=ab
.
Ed