5 real

Algebra Level 3

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?

Yes, it is possible No, it is not possible

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3 solutions

Shaun Leong
Sep 6, 2017

4 5 6 7 ( 2 ) = 5 6 7 8 ( 1 ) 4*5*6*7*(-2)=5*6*7*8*(-1)

Choose any 4 real numbers a , b , c , d a,b,c,d such that none of them is equal to zero or 1 -1 .

Then the fifth number can be calculated as

e = ( 1 + a ) ( 1 + b ) ( 1 + c ) ( 1 + d ) a b c d ( 1 + a ) ( 1 + b ) ( 1 + c ) ( 1 + d ) e = \frac{(1 + a) (1 + b) (1 + c) (1 + d)}{a b c 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 ) abcd=(1+a)(1+b)(1+c)(1+d)

Samir Betmouni
Sep 9, 2017

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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