## mathnoob1 2 years ago show cos2x = 1 - tan^2 (x) / 1 +tan^2 (x)

1. Nikhil619

do u know how to start?

2. mathnoob1

|dw:1359466433151:dw|

3. mathnoob1

|dw:1359466643501:dw|

4. Nikhil619

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

5. Nikhil619

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

6. mathnoob1

sinx/cosx?

7. Nikhil619

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

8. mathnoob1

ya

9. Nikhil619

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

10. mathnoob1

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

11. Nikhil619

take lcm..

12. Nikhil619

Simplify more...

13. Tushara

i hope dis helps

14. Nikhil619

@mathnoob1 u got it?

15. mathnoob1

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

16. Nikhil619

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

17. mathnoob1

any tips to know which identity fit most?

18. Nikhil619

there aren't any methods or anything..see u have 2 sides of a proof right..one side will be some complicated thing...while the other will probably be sumthin u know the direct equation for like here cos2x=cosx-sinx...this u already know...so try converting the complicated thing in terms of the other side equation....

19. mathnoob1

ok

20. mathnoob1

i got it cos2x/1 = cos2x