4 1*39=(40+ 1)*(40- 1)= 1599= 1600- 1* 1

42*38=(40+2)*(40-2)= 1596= 1600-2*2

43*37=(40+3)*(40-3)= 159 1= 1600-3*3

Multiplied by the multiplier of forty plus one and the multiplier of forty minus one, the result is the difference between the product of forty and forty and the product of addition and subtraction.