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 Trigonometry Trigonometry Math Forum

 May 9th, 2019, 01:04 PM #1 Newbie   Joined: Apr 2019 From: Malawi Posts: 19 Thanks: 0 Math Focus: Trigonometry and calculus Trigonometric Quadrant Suppose that A is a second quadrant angle with sin A = 1/4. Determine the exact value of sin(A/2). The problem has made me crazy. Help please Last edited by Khoxy; May 9th, 2019 at 01:11 PM. May 9th, 2019, 01:44 PM #2 Global Moderator   Joined: May 2007 Posts: 6,806 Thanks: 716 $\sin\frac{A}{2}=\sqrt{\frac{1-\cos A}{2}}$ where $\cos A=-\sqrt{\frac{15}{16}}$. The sign for $\cos A$ is negative, since $A$ is in the second quadrant. Ans.=0.984250984251476 Last edited by skipjack; May 9th, 2019 at 01:59 PM. May 9th, 2019, 02:02 PM #3 Newbie   Joined: Apr 2019 From: Malawi Posts: 19 Thanks: 0 Math Focus: Trigonometry and calculus Mathman I don't understand how you come up with cos A. Would you clarify? May 9th, 2019, 02:10 PM #4 Global Moderator   Joined: Dec 2006 Posts: 20,921 Thanks: 2203 cos²(A) ≡ 1 - sin²(A) and cos(A) ≡ 1 - 2sin²(A/2) $\sqrt{\small8 + 2\sqrt{15}}/\small4 = 0.9920$... May 10th, 2019, 12:49 PM #5 Global Moderator   Joined: May 2007 Posts: 6,806 Thanks: 716 $\sin{\frac{A}{2}}=0.992029696267167$ Arith. error in previous. $\cos^2A=1-\sin^2A=1-\frac{1}{16}=\frac{15}{16}$. Last edited by skipjack; May 10th, 2019 at 10:35 PM. Tags geometric, quadrant Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post jayinwww Algebra 2 November 25th, 2016 10:21 PM helpme93 Elementary Math 3 June 26th, 2014 12:03 PM yaantey Trigonometry 2 November 29th, 2013 02:06 AM unwisetome3 Calculus 7 February 8th, 2013 07:55 PM mrcode Advanced Statistics 0 March 18th, 2011 10:52 AM

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