左边=1/[(sin^2)a(cos^)a]-(cos^2)a/(sin^2)a=[1-(cos^4)a]/[(sin^2)a(cos^2)a]=[1+(cos^2)a] [1-(cos^2)a]/[(sin^2)a(cos^2)a]=[1+(cos^2)a]/(cos^2)a= [(sin^2)a+(cos^2)a+(cos^2)a]/(cos^2)a=[(sin^2)a+2(cos^2)a...