(1/3a^3-5a^4b+1/2a^5)÷(-2/3a^3)
=(1/3a^3)÷(-2/3a^3)-5a^4b÷(-2/3a^3)+1/2a^5)÷(-2/3a^3)
=-1/2+15/2ab-3/4a^2