p.t A,B,C are interior

Question

p.t A,B,C are interior <s of tri.ABC, show that cos^2A/2+cos^2(B+c)/2=1

anonymous · Answer

@ash2326

ash2326 · Answer

we know that
$$A+B+C=180$$
do you agree with this?

anonymous · Answer

yaa

ash2326 · Answer

so
$$A=180-(B+C)$$
Take cos both sides
$$\cos A=\cos(180-(B+C))$$
could you simplify this, right side?

anonymous · Answer

yaa ok thx

ash2326 · Answer

it's not over, could you simplify?
what's 
$$\cos(180-x)$$?

anonymous · Answer

noooooooo

ash2326 · Answer

cos (180-x)=-cos x
so
we get
$$\cos A=-\cos (B+C)$$
do you know the formula for $\cos (2\theta)$ in terms of $\cos \theta$?
also known as the half angle cosine formula!

anonymous · Answer

nooo

ash2326 · Answer

@Parvathysubhash I think you need to study these topics, particularly half angle formulas and the values and signs of trigonometric functions.
Without this basic knowledge, you can't understand this question. Even if I solve it for you, it won't be of much help to you. Please study these topics, I could give you links to sites, where you'd be able to learn these topics.

anonymous · Answer

kkk

ash2326 · Answer

Please see these links: http://www.clarku.edu/~djoyce/trig/ http://www.learnerstv.com/Free-Maths-Video-lectures-ltv096-Page1.htm Thanks