Mathematical puzzle 123 If C is a positive integer, and a + B = C, B + C = D, D + a = B, then what is the minimum value of (a + b) (B + C) (c + D) (D + a)?

Mathematical puzzle 123 If C is a positive integer, and a + B = C, B + C = D, D + a = B, then what is the minimum value of (a + b) (B + C) (c + D) (D + a)?

According to a + B = C, B + C = D, D + a = B, we can treat a, B, D as unknowns, and C as a parameter
a= -c
b= 2c
d= 3c
So (a + b) (B + C) (c + D) (D + a) = 24C ^ 4 (the fourth power of C)
Because C is a positive integer, the minimum value of the original formula is 24