algubra

Expand the following:
(r-4) (r+4)

 

(r-4) (r+4) = (r) (r) + (r) (4) + (-4) (r) + (-4) (4):
r r+4 r-4 r-4 4

 

r r = r^2:
r^2+4 r-4 r-4 4

 

-4×4  =  -16:
r^2+4 r-4 r+-16

 

4 r-4 r = 0:
Answer: |  r^2-16