leo324 Posted May 20, 2019 Report Share Posted May 20, 2019 Given that sinx = 5/13 and x is an obtuse angle. determine the exact value of sin2x. Reply Link to post Share on other sites More sharing options...
kw0573 Posted May 21, 2019 Report Share Posted May 21, 2019 The wrong approach is to solve for x. The right approach is to apply the double angle identity (sin 2x = 2(sin x)(cos x)). For that, we need to know cos x, which by default is calculated via the pythagorean identity (sin² x + cos² x = 1) whenever we need to convert between sine and cosine of the same angle. cosine is negative in the second quadrant (obtuse angles), ie. cos x < 0 (5/13)² + cos² x = 1, cos x = - sqrt ((169-25)/169) = -12/13 Finally, sin 2x = 2(5/13)(-12/13) = -120/169 Reply Link to post Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.