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do u know how to start?

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Umm...dont do that...u'll only get cos2x=1...

what is tanx=?i.e what can tanx be written as?

sinx/cosx?

yeah good..so tan^2x = sin^2 x/cos^2 x ?

ya

so nw substitute...and tell me what u get..

1-(sin(^2)x/cos(^2)x /
1+(sin(^2)x/cos(^2)x ?

take lcm..

Simplify more...

i hope dis helps

@mathnoob1 u got it?

do we use tan^2+1=sec^2
and 1+cot^2 =csc^2 for proving?

U can use any Identity for proving but here as far as I know only this will work...

any tips to know which identity fit most?

ok

i got it cos2x/1 = cos2x