Posted 07 January 2012 - 05:27 PM
Find all the values of x in the interval 0 (which is less than or equal to ) x (which is less than or equal to ) 2 pi for which sin2x = -cosx
Please help, thanks
Posted 07 January 2012 - 06:23 PM
(see if you can do this, hint use the sin(2x) expansion)
so our solutions occur when cos(x)=0 or 2sin(x)+1=0 (ie sin(x)=-0.5)
which from our sine and cosine plots, and knowing sin(30)=0.5 we get solutions of
90, 210, 270, 330 degrees (or π/2, 7π/6, 3π/2, 11π/6 in rads)
Hope this helps, and if you have any further problems just reply
Posted 07 January 2012 - 06:42 PM
Thanks a lot for your help. I'm a bit confused about the sin(2x) expansion. How would I do that?
Posted 07 January 2012 - 07:38 PM
and take out a common factor of cos(x)
NOTE: we can't just cancel the cosines in the first line, because that makes the assumption that cos(x)/cos(x)=1, which is not necessarily the case for cos(x)=0 (0/0=???) which unfortunately are solutions for this particular problem so if you try that you don't get every solution to this problem.
1 user(s) are reading this topic
0 members, 1 guests, 0 anonymous users