## tanjung 3 years ago Help... Prove that sin36 degrees = sqrt[10 - 2*sqrt(5)]/4

1. micahwood50

HINT: 180º/5 = 36º

2. tanjung

next, what's formula using i do

3. micahwood50

Seems like it won't help. Try this, using the formula sin (a + b) = cosa sinb + cosb sin a sin36º = 2 sin18º cos18º = 2 cos72º sin72º

4. tanjung

ok, i know that formula.. but i think we get the new problem... because sin/cos 18 or sin/cos 72 not eassy to get.. it's very difficul, right ?

5. micahwood50

It seems so. Bump your question to get more helps. Maybe someone can help you with this.

6. tanjung

ok, thanks...

7. tanjung

@cwrw238 , @experimentX , @estudier , @Hero , @Zarkon , @sauravshakya , @all... help me please, how to get it

8. tanjung

any idea guys ?

9. ajprincess

|dw:1350231015827:dw||dw:1350231152168:dw| Use quadratic formula |dw:1350231253477:dw| |dw:1350231349862:dw|

10. experimentX

@ajprincess try this|dw:1350231460480:dw|

11. tanjung

ok, i get |dw:1350231717622:dw|

12. tanjung

next ?

13. tanjung

upss. sorry, divided 8

14. ajprincess

|dw:1350231971343:dw||dw:1350232035700:dw|

15. ajprincess

@experimentX I tried bt it is a bit complex. cos sin3x=sin108.

16. ajprincess

Getting it @tanjung?

17. experimentX

this should be simpler ... sin(3x) = sin(2x)

18. tanjung

not yet, what should i do after that

19. experimentX

since this is just double angle and triple angle ... change everything back to sin ... and remove all roots and try to factorize it.

20. ajprincess

|dw:1350232254804:dw|

21. tanjung

22. tanjung

i know that +- it means there are two number here. what about u for it

23. mukushla

with positive sign u have sin 36>1 thats impossible

24. ajprincess

|dw:1350233305717:dw|

25. tanjung

oh yea, i understand now... thank u very much for ur help , @ajprincess and @mukushla

26. ajprincess

yw:)