\(The\ scientific\ notation:a\cdot10^n\ where\ 1\leq a \ \textless \ 10\ and\ n\in\mathbb{Z}\\\\8.\\3.4(10^{-4})+0.8(10^{-4})=(3.4+0.8)(10^{-4})=\boxed{4.2\cdot10^{-4}}\\\\9.\\8.7(10^{-2})-2.3(10^{-2})=(8.7-2.3)(10^{-2})=\boxed{6.4\cdot10^{-2}}\\\\10.\\6.3(10^5):1.5(10^{-3})=(6.3:1.5)(10^5:10^{-3})\\\\=4.2\cdot10^{5-(-3)}=\boxed{4.2\cdot10^8}\)
