TRIGONOMETRY

How do I prove that tan70×tan20=1?
tan90=tan(70+20)=(tan70+tan20)/(1-tan70tan20)

but tan90=∞=1/0

or

1/0=tan90=tan(70+20)=(tan70+tan20)/(1-tan70tan20)

solving

0=(1-tan70tan20)

tan70tan20=1 

hence proved

What is the value of Sin 15+Cos 15?
sin15+cos15 =I

squring both sides

sin²15+cos²15+2sin15cos15=I²

1+sin30=I²

1+(1/2)=I²

3/2=I²

I=√(3/2)