(1) 99.8^2=(100-0.2)^2=100^2 - 2*100*0.2 + 0.2^2 = 10000-40+0.04=9960.04
(2)(a+b)(a-b)(a^2-b^2)=(a^2-b^2)^2=a^4 - 2*a^2*b^2+b^4