Final answer:
To illustrate the identity property of multiplication and maintain the value of 57 in the equation, the complete number sentence is 1 x 57 = 57.
Step-by-step explanation:
The question asks us to complete a number sentence to illustrate the identity property of multiplication. This property states that any number multiplied by 1 equals the number itself. Therefore, to maintain the value 57 in the equation, one of the factors needs to be 1, and the other factor should be 57. The complete number sentence that illustrates the identity property of multiplication is:
1 x 57 = 57