Make This Solvable

All variables are positive integers. Which variable must be given so that the equation can be solved?

a b + a c = a b c \large ab + ac = abc

a a b b c c

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2 solutions

Given that a b + a c = a b c a ( b + c ) = a ( b c ) ab+ac=abc \iff a(b+c)=a(bc)

Now we need to remove a a on both sides. Unfortunately, no variable has provided condition, while to eliminate a a , we need a 0 a \neq 0 so that the equation above solvable.

given a 0 a\neq 0

a ( b + c ) = a b c b + c = b c b ( 1 c ) = c a(b+c)=abc \implies b+c=bc \implies b(1-c)=-c

since c c and 1 c 1-c are relatively primes, 1 c = 1 c = 2 b = 2 1-c= -1 \implies c=2 \implies b=2

So, given a a can be eliminated, b , c b,c can be determined uniquely. Therefore, a a should be given.

Thank youo veri mucho!

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