28. 已知各项都为正数的等比数列{an}\{a_{n}\}{an},若a8⋅a12+5a10=14a_{8}\centerdot a_{12}+5a_{10}=14a8⋅a12+5a10=14,则log2a1+log2a2+log2a3+…+log2a19=\log _{2}a_{1}+\log _{2}a_{2}+\log _{2}a_{3}+\ldots +\log _{2}a_{19}=log2a1+log2a2+log2a3+…+log2a19= ;