To find the integral of cos 2 x, we use the double angle formula of cos. One of the cos 2x formulas is cos 2x = 2 cos 2 x - 1. By adding 1 on both sides, we get 1 + cos 2x = 2 cos 2 x. By dividing by both sides by 2, we get cos 2 x = (1 + cos 2x) / 2. We use this to find ∫ cos 2 x dx. Then we get. ∫ cos 2 x dx = ∫ (1 + cos 2x) / 2 dx.
$\begingroup$ Why: $$\cos ^2(2x) = \frac{1}{2}(1+\cos (4x))$$ I don't understand this, how I must to multiply two trigonometric functions? Thanks a lot. asked Oct 28, 2012 at 1:54 $\endgroup$ 2 $\begingroup$ Recall the formula $$\cos(2 \theta) = 2 \cos^2(\theta) - 1$$ This gives us $$\cos^2(\theta) = \dfrac{1+\cos(2 \theta)}{2}$$ Plug in $\theta = 2x$, to get what you want. EDIT The identity $$\cos(2 \theta) = 2 \cos^2(\theta) - 1$$ can be derived from $$\cos(A+B) = \cos(A) \cos(B) - \sin(A) \sin(B)$$ Setting $A = B = \theta$, we get that $$\cos(2\theta) = \cos^2(\theta) - \sin^2(\theta) = \cos^2(\theta) - (1-\cos^2(\theta)) = 2 \cos^2(\theta) - 1$$ answered Oct 28, 2012 at 1:56 $\endgroup$ 1 $\begingroup$It’s just the double-angle formula for the cosine: for any angle $\alpha$, $\cos 2\alpha=\cos^2\alpha-\sin^2\alpha\;,$ and since $\sin^2\alpha=1-\cos^\alpha$, this can also be written $\cos2\alpha=2\cos^2\alpha-1$. Now let $\alpha=2x$: you get $\cos4x=2\cos^22x-1$, so $\cos^22x=\frac12(\cos4x+1)$. answered Oct 28, 2012 at 1:57 Brian M. ScottBrian M. Scott590k52 gold badges711 silver badges1179 bronze badges $\endgroup$ 1 $\begingroup$$$\cos(4x) = \cos^2 (2x) - \sin^2 (2x) = 2\cos^2 (2x) - 1$$ answered Oct 28, 2012 at 1:57 InquestInquest6,4472 gold badges32 silver badges56 bronze badges $\endgroup$ 0 Not the answer you're looking for? Browse other questions tagged algebra-precalculus trigonometry or ask your own question.
From the double angle identities, cos2x −1 = cos2x: = − 1( − 1 + cos2x) Finally: 1 − cos2x = 1 − cos2x. We have proved that 1 − cos2x = tanxsin2x. Hope this helps! Answer link. Here's how I proved it: 1-cos2x =tanxsin2x I'll prove using the right hand side of the equation. From the double angle identities, sin2x=2sinxcosx So for this question you can use either the product rule or the quotient rule and I'll run through them the quotient rule:The quotient rule says that if you have h(x)=f(x)/g(x)Then h'(x) = (f'(x)g(x)-f(x)g'(x))/(g(x))^2So using f(x)=cos(2x) and g(x)=x^1/2then f'(x)=-2sin(2x) and g'(x)=1/2x^-1/2Plugging this into our formula gives ush(x) = (-2x^1/2sin(2x)-1/2x^-1/2cos(2x))/xAlways remember to simplify afterwards which gives us(-2x^1/2sin(2x)-1/2x^-1/2cos(2x))/xSecond the product rule:What the product rule says is that ifh(x) = f(x)g(x)then h'(x) = f(x)g'(x) + f'(x)g(x)So if we say that h(x) = cos(2x)/x^1/2Then we can say that f(x) = cos(2x) and g(x) = x^-1/2Using the product rule we have:f(x) = cos(2x) f'(x) = -2sin(2x)g(x) = x^-1/2 g'(x) = 1/2x^-3/2So lastly we know that h(x) = f(x)g'(x) + f'(x)g(x)So using what we've found out we can say that h(x) = (cos(2x))/(2x^3/2)-(2sin(2x))/x^1/2Once again simplifying gives us(-2x^1/2sin(2x)-1/2x^-1/2cos(2x))/xNeed help with Maths?One to one online tuition can be a great way to brush up on your Maths a Free Meeting with one of our hand picked tutors from the UK’s top universitiesFind a tutor 5Edis.