2.21. Trang 42

A = $4^{4}$.$9^{5}$

  = 4.4.4.4.9.9.9.9.9

  = $2^{2}$. $2^{2}$. $2^{2}$. $2^{2}$.$3^{2}$.$3^{2}$.$3^{2}$.$3^{2}$.$3^{2}$

  = $2^{2+2+2+2}$.$3^{2+2+2+2}$

  = $2^{8}$.$3^{10}$