# Prove that in any triangle $ABC$, $\cos^2A+\cos^2B+\cos^2C\geq\frac{3}{4}$

I have two similar looking questions.

(1) Prove that in triangle $ABC$, $$\cos^2A+\cos^2B+\cos^2C\geq\frac{3}{4}.$$

(2) If $\Delta ABC$ is an acute angled, then prove that
$$\cos^2A+\cos^2B+\cos^2C<\frac{3}{2}$$

If I apply Jensen's inequality, then $\cos^2x$ is a concave function, because its second derivative is $-2\cos 2x$ and with it being concave function $$\cos^2A+\cos^2B+\cos^2C\leq\frac{3}{4}$$ which is not there in the question.How will we prove both of these questions. I have some intuition that in the second question, as $ABC$ is an acute angled triangle,this has something to do.

Please guide me in the right direction. Thank you.

• $-2\cos(2x)$ is not always non-negative, even for angles of a triangle. – Quang Hoang Sep 3 '15 at 12:44
• It isn't true that $cos^2(A)+cos^2(B)+cos^2(C)≤\frac 34$. For the right isosceles triangle you get $1$, which I believe is the upper bound in the acute case. – lulu Sep 3 '15 at 12:53

$$y=\cos^2A+\cos^2B+\cos^2C=\cos^2A-\sin^2B+\cos^2C+1$$

Now $\cos^2A-\sin^2B=\cos(A+B)\cos(A-B)=-\cos C\cos(A-B)$

$$\implies\cos^2C-\cos C\cos(A-B)+1-y=0$$ which is a Quadratic Equation in $\cos C$

$$\implies\cos^2(A-B)-4(1-y)\ge0\iff4y\ge4-\cos^2(A-B)=3+\sin^2(A-B)$$

$$\implies4y\ge3$$

The equality occurs if $\sin(A-B)=0\implies A=B\ \ \ \ (1)$ and $\cos C=\dfrac12\implies C=\dfrac\pi3\ \ \ \ (2)$

$(1),(2)\implies A=B=C$

$$\cos^2A+\cos^2B+\cos^2C=1+\cos^2C-\cos C\cos(A-B)$$

$$=1+\cos C[\cos C-\cos(A-B)]$$

$$=1-\cos C[\cos(A+B)+\cos(A-B)]$$

$$=1-2\cos A\cos B\cos C<1$$ if $0<A,B,C<\dfrac\pi2$

• How will we prove second part,Sir @lab bhattacharjee. – user1442 Sep 3 '15 at 13:00
• @user1442, Please wait before accepting :) – lab bhattacharjee Sep 3 '15 at 13:02

Multiplying by $4R^2$ and exploiting the sine theorem we get:

$$4R^2\sum_{cyc}\cos^2 A = 12R^2-(a^2+b^2+c^2)$$ hence the first inequality is equivalent to the trivial $OH^2\geq 0$, where $O$ is the circumcenter and $H$ is the orthocenter. On the other hand, if $ABC$ is an acute-angled triangle we have that $H$ lies inside $ABC$, hence $OH^2< R^2$ and the second inequality follows.

Eliminate $C$ from $A+B+C= \pi$

Next, recognize the bounds of $\cos A, \cos B.$

• Cant it be proved by Jensen,Sir? – user1442 Sep 3 '15 at 12:40
• Sketching $\cos x ^2$ It is both concave and convex in different parts. Show the left and right hand sides if it is convex. – Narasimham Sep 3 '15 at 12:51
• It does not look as an answer to me. – Jack D'Aurizio Sep 3 '15 at 13:21
• Yes, I wanted to expand it further, OP seemed to already catch that point.. – Narasimham Sep 4 '15 at 6:20

Try the first one. Transform the triangle using linear transformations(thereby keeping angles invariant) to A(0,a),B(-b,0),C(c,0). What we are after is bb/(aa+bb) +cc/(aa+cc)+(aa/(sqrt(aa+cc)sqrt(aa+bb))-cb/(sqrt(aa+cc).sqrt(aa+bb)))^2>=3/4. Simplifying then we are after 4(3bbcc+aaaa+bbaa+aacc)>=3(aaaa+aabb+aacc+bbcc). This is clear.

• You have enough reputation to understand it is better to give answers by using the $\LaTeX$ support. – Jack D'Aurizio Sep 3 '15 at 13:47