... Дальше сами
<span>tg 55° - tg 35° =[sin55cos35-cos55sin35]/cos55cos35=</span><span>sin[55-35]/1/2*[cos20+cos90]=</span><span>2sin20/cos20=2tg20</span>
cosa-sqrt)2)(cosa*sqrt(2)/2-sinasqrt(2)/2)=cosa-cosa+sina=sina