1)1+(5tgx/2+4)²/9=(9+25tg²x/2+40tgx/2+16)/9=
=(25tg²x/2+25+40tgx/2)/9=(25/cos²x/2+40sinx/2/(cosx/2))/9=
=(25+40sinx/2cosx/2)/9cos²x/2=(25+20sinx)/9cos²x/2
2)1:(25+20sinx)/9cos²x/2=(9cos²x/2)/(5(5+4sinx))
3)5/9*1/cos²x/2*(9cos²x/2)/(5(5+4sinx))=1/(5+4sinx)