Answer 1

$tg105 ^{0} =tg(90 ^{0} +15 ^{0} )=-ctg15 ^{0}$

Answer 2

tg105=tg(60+45)=(tg60+tg45)/(1-tg60*tg45)=(√3+1)/(1-√3)=(√3+1)²/(1-3)=
=(4+2√3)/(-2)=-2-√3