You can put this solution on YOUR website! Cos2A = Cos (A+A) we know the formula for Cos (A+B)=CosACosB-SinASinB therefore Cos (A+A)= CosACosA - SinASinA = Cos^2A - Sin^2A WE also know that Cos^2A + Sin^2A = 1 therfore Sin^2A = 1 - Cos^2A Replacing the value of Sin^2A Cos (A+A)= Cos^2A - (1 - Cos^2A) Cos2A = Cos^2A - 1 +Cos^2A Hence Cos2A = 2Cos^2A - …

What are the steps to prove that this equation is and identity? $$\frac {\cot A - \tan A} {\cot A + \tan A}=\cos2A$$

2019年8月6日 · I know the solution to this question, which is very long. Prove the trigonometric identity $$\cos2A+\cos2B-\cos2C=1-4\sin A\sin B\cos C \tag {$\star$}$$ where the angles are part of $\triangle...

2015年1月12日 · Let the three internal angles of a triangle are a, b, c. Prove that cos(2a) + cos(2b) + cos(2c) ≥ − 3 2.

2018年10月28日 · Given that $2\\cos(3a)=\\cos(a)$ find $\\cos(2a)$. $2\\cos(3a)=\\cos(a)$ I converted $\\cos(2a)$ into $\\cos^2(a)-\\sin^2(a)$ Then I tried plugging in. I know this is not right, but I have no clue how to

2015年4月20日 · Probably it is not very much as a "proof" but... if A=30° 2A=60° so: cos (60°)=1/2 2cos^2 (30°)-1=2 (sqrt (3)/2)^2-1=3/2-1=1/2

2018年3月9日 · Prove cos2 A + sin2 A ⋅ cos 2B = cos2 B + sin2 B ⋅ cos 2A?

2019年9月10日 · There is a trigonometric identity: $$\cos^2A+\cos^2B+\cos^2C+2\cos A\cos B\cos C\equiv 1\text { when }A+B+C=\pi$$ It is easy to prove it in an algebraic way, just like that: Then, I want to fin...

Question 1032544: If ∠A is ACUTE and cos A = 4/5 find sin A, cos2A, sin2A, tan2A Answer by ikleyn (52345) (Show Source):

How can I prove this equation? 1 + cos 2C − cos 2A − cos 2B = 4 sin A sin B sin C 1 + cos 2 C − cos 2 A − cos 2 B = 4 sin A sin B sin C if we know that A A, B B, C C are a triangle's angles. I have come to the point where on the left side I have −4 cos A cos B cos C − 4 cos A cos B cos C. On the other side I have 4 sin A sin B sin C 4 sin A sin B sin C. Am I correct till now?