解:$(1)10^8 ×10^4 = 10^{8+4} = 10^{12}$
$(2)(-x)\ \mathrm {·}(-x)^2\ \mathrm {·}(-x)^3 = (-x)^{1+2+3} = (-x)^6 = x^6$
$(3)a^{n+2}\ \mathrm {·}a^{n+1}\ \mathrm {·}a^n ·a = a^{(n+2)+(n+1)+n+1} = a^{3n+4}$
$(4)(b+2)^3\ \mathrm {·}(b+2)^5\ \mathrm {·}(b+2) = (b+2)^{3+5+1} = (b+2)^9$