Proving. Sec 4 A Maths Help

• galaxy_twirls

  25 Feb 12, 07:29

  Hi everyone.

  I need help on an A Maths question which I'm super stuck at and desperately need help. T_T 

   

  How do you prove:

   

  4 sin^2 (x) - 7sin x cos x - 5 cos^3 (x) = -2

   

  I have tried applying trig identities but I can't seem to get anything... From the above, it means that the sines and cosines have to cancel out to yield a constant right? Can anyone guide me to see the pattern in the above? Thanks! :D

• wee_ws

  25 Feb 12, 09:03

  Hi,

    Don't worry, this question is problematic in the first place.

    It is easy to provide a counterexample (let x = 0) to show that the equation does not hold.

    Thanks.

  Cheers,
  Wen Shih

• galaxy_twirls

  25 Feb 12, 10:07

  Hi Wen Shih,

   

  Thank you for you help! Oh dear, why didn't I check it? >< My apologies for posting this. :X Thanks for your guidance again! :D

   

  Regards,

  galaxy_twirls