# Trig identities plz help

by nelraheb
Tags: identities, trig
 P: 6 In triangle ABC if sin (A/2) sin (B/2) sin (C/2) = 1/8 prove that the triangle is equilateral plz show steps
 Sci Advisor HW Helper PF Gold P: 562 Perhaps you can use the sine rule (?)
 P: 6 Sure .. I tried but had no success .. If you find an answer plz post your steps
P: 137

## Trig identities plz help

one method suggested:

(an absurd reasonning)

if it is an equilateral triangle then:
A = B = C = pi/3 rad

implies ---> A/2 = B/2 = C/2 = pi/6 rad

implies ---> sin(A/2) = sin(B/2) = sin(C/2) = 1/2

implies ---> sin(A/2)sin(B/2)sin(C/2) = 1/2*1/2*1/2 = 1/8

thus it is indeed an equilateral triangle

if i come with another one i will post it :)
hope it will help
 HW Helper P: 1,422 Try expand the equation $$\sin{\frac{A}{2}}\sin{\frac{B}{2}}\sin{\frac{C}{2}} = \frac{1}{8}$$ to $$4\sin{\frac{C}{2}}^{2} - 4\sin{\frac{C}{2}}\cos{\frac{A - B}{2}} + 1 = 0$$ Then to $$(2\sin{\frac{C}{2}} - \cos{\frac{A - B}{2}})^{2} + (\sin{\frac{A - B}{2}})^{2} = 0$$ Now you have something like $A^{2} + B^{2} = 0$ so $$\left\{ \begin{array}{c} A = B \\ \sin{\frac{C}{2}} = \frac{1}{2}\cos{\frac{A - B}{2}} \end{array}\right$$ So you will have A = B = C = 60 degrees, which implies the triangle ABC is equilateral. Hope it help. Viet Dao,
 P: 6 For AI thank you but this won't do
 P: 6 For VietDao29 If A^2 + B^2 = 0 then we're stuck because no two +ve numbers addto zero ...right ? Then it should be A^2 = - B^2 How did you expand 1st step How did you get last step plz go in more details
 Sci Advisor HW Helper P: 11,835 Both A and B are real.So their square is larger or equal to zero.In order for the sum of the squares to be 0,each if the squares must be 0. Daniel.
 P: 6 Well that's a good point. How did I miss that :) Now for the first step plz how did we expand Sin (A/2) Sin (B/2) Sin (C/2) to next step ie. How to start .... the rest is ok
 Sci Advisor HW Helper P: 11,835 Use this IDENTITY: $$\sin x\sin y\equiv \frac{1}{2}[\cos(x-y)-\cos(x+y)]$$ The result is immediate. Daniel.
 Emeritus Sci Advisor PF Gold P: 5,540 Try starting by eliminating a variable. Since you know that A, B, and C are all in the same triangle, you have: A+B+C=180 C=180-A-B See where that gets you.
 P: 6 Thank you all ... I can do it now following your steps The rule supplied by Dextercioby did not look familiar (but it's correct I checked) ..well memory is not what it used to be :) isn't that a bit complicated though ... I thought the answer should be more straight forward .. any way thank you all again