1.4sinA-2cosA/5cosA+3sinA
=4-2tanA/5+3tanA (上下两边同除以cosA)
=4-2*2/5+3*2
=0
2.sinAcosA=sinAcosA/sinA²+cosA²
=tanA/tanA²+1（上下两边同除以 cosA²）
=2/2²+1
=2/5