trigonometry - How prove this $cos{x}+cos{y}+cos{z}=1$

Question:

let $x,y,z\in R$ and such $x+y+z=\pi$,and such

$$\tan{\dfrac{y+z-x}{4}}+\tan{\dfrac{x+z-y}{4}}+\tan{\dfrac{x+y-z}{4}}=1$$
show that
$$\cos{x}+\cos{y}+\cos{z}=1$$

My idea: let

$$x+y-z=a,x+z-y=b,y+z-x=c$$
then
$$a+b+c=\pi$$
and
$$\tan{\dfrac{a}{4}}+\tan{\dfrac{b}{4}}+\tan{\dfrac{c}{4}}=1$$
we only prove
$$\cos{\dfrac{b+c}{2}}+\cos{\dfrac{a+c}{2}}+\cos{\dfrac{a+b}{2}}=1$$
Use
$$\cos{\dfrac{\pi-x}{2}}=\sin{\dfrac{x}{2}}$$
$$\Longleftrightarrow \sin{\dfrac{a}{2}}+\sin{\dfrac{b}{2}}+\sin{\dfrac{c}{2}}=1$$

let
$$\tan{\dfrac{a}{4}}=A,\tan{\dfrac{b}{4}}=B,\tan{\dfrac{\pi}{4}}=C$$
then
$$A+B+C=1$$
and use $$\sin{2x}=\dfrac{2\tan{x}}{1+\tan^2{x}}$$
so we only prove
$$\dfrac{2A}{1+A^2}+\dfrac{2B}{1+B^2}+\dfrac{2C}{1+C^2}=1$$

other idea:let

$$\dfrac{y+z-x}{4}=a,\dfrac{x+z-y}{4}=b,\dfrac{x+y-z}{4}=c$$

then we have
$$a+b+c=\dfrac{\pi}{4},\tan{a}+\tan{b}+\tan{c}=1$$
we only prove
$$\cos{(2(b+c)}+\cos{2(a+c)}+\cos{2(a+b)}=\sin{(2a)}+\sin{(2b)}+\sin{(2c)}=1$$
then I fell very ugly, can you some can help?

Thank you very much!