Cos420°= Tg(-60°)= (1-sin^2t)(1+tg^2)=1

[latex]cos 420 = cos (2n + 60^{0}) = cos 60^{0}= \frac{ 1 }{2} [/latex] [latex]tg (-60)= - tg60=- \sqrt{3} [/latex] [latex](1-sin^{2}t) (1+tg^{2}t)=1[/latex] [latex](sin^{2}t+cos^{2}t-sin^{2}t) ( \frac{1}{cos^{2}} )=1[/latex] [latex] ( \frac{cos^{2}}{cos^{2}} )=1[/latex] 1=1