[tex]\displaystyle\bf\\ c)\\3^9+6\cdot3^8=3^{8+1}+6\cdot3^8=3^8\cdot3+6\cdot3^8=3^8(3+6)=3^8\cdot3^2=\boxed{\bf3^{10}}\\\\d)\\2^{10}-2^9=2^{9+1}-2^9=2^{9}\times2-2^9=2^9(2-1)=\boxed{\bf2^9}\\\\e)\\3^{10}+2\cdot3^{10}=3^{10}(1+2)=3^{10}\cdot3=\boxed{\bf3^{11}}\\\\f)\\3\cdot2^{20}-2\cdot2^{20}=2^{20}(3-2)=2^{20}\cdot1=\boxed{\bf2^{20}}[/tex]
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[tex]\displaystyle\bf\\g)\\7\cdot3^{15}+20\cdot3^{15}=3^{15}(7+20)=3^{15}\cdot27=3^{15}\cdot3^3=3^{15+3}=\boxed{\bf3^{18}}\\\\h)\\3\cdot5^{12}+2\cdot5^{12}=5^{12}(3+2)=5^{12}\cdot5=5^{12+1}=\boxed{\bf5^{13}}\\\\i)\\3^{20}-2\cdot3^{19}=3^{19+1}-2\cdot3^{19}=3^{19}\cdot3-2\cdot3^{19}=3^{19}(3-2) =\boxed{\bf3^{19}}[/tex]
