Exercise 57 Page 32

1. (2c-d)(2c-d)
2. (2c+d)(2c-d)
3. (2c+d)(2c+d)
4. (c+d)(c+d)

Looking at the expressions, 1, 3, and 4 have matching expressions meaning they can be expressed as the square of an expression.

1. (2c-d)(2c-d) = (2c-d)^2
2. (2c+d)(2c-d)
3. (2c+d)(2c+d)= (2c+d)^2
4. (c+d)(c+d) = (c+d)^2

We can say (2c+d)(2c-d) does not belong because it cannot be represented as a square of an expression.