Question 174067
OK, that is so:

(a-b)*(a+b)=a^2+ab-ab-b^2 ; ab-ab=0 , so (a-b)*(a+b)=a^2-b^2. 

In this case b is always b=1 so:

(a-1)*(a+1)=a^2-1^2; 1^2=1, (a-1)*(a+b)=a^2-1.

Now when a=40, b=1, 
(40-1)*(40+1)=40^2-1^2;
(60-1)*(60+1)=60^2-1^2  etc.....

I hope you like the answer and I am sorry for my bad English.