I have 5 real numbers whose product is non-zero. Now, I increase each of the 5 numbers by 1 and again multiply all of them. Is it possible that this new product is the same as the non-zero number obtained earlier?
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Choose any 4 real numbers a , b , c , d such that none of them is equal to zero or − 1 .
Then the fifth number can be calculated as
e = a b c d − ( 1 + a ) ( 1 + b ) ( 1 + c ) ( 1 + d ) ( 1 + a ) ( 1 + b ) ( 1 + c ) ( 1 + d )
Of course, you need to avoid a choice which results in a b c d = ( 1 + a ) ( 1 + b ) ( 1 + c ) ( 1 + d )
I split it into choosing three numbers, and two numbers. Each set having this quality with products.
So: ab = (a+1)(b+1) = ab + a + b + 1
Which requires a+b = -1
By a similar argument:
x,y,z can be achieved with y = -z and y^2 = x + 1
Eg: let a,b,x,y,z be
1,-2,3,2,-2 with product 24
While increments: 2,-1,4,3,-1 also have product 24
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4 ∗ 5 ∗ 6 ∗ 7 ∗ ( − 2 ) = 5 ∗ 6 ∗ 7 ∗ 8 ∗ ( − 1 )