I’m trying to do a proof using trig identities, I get a fail, and I,m not sure if I’m using @test wrong, or if I made a mistake in my math s.t.

cos(x)-cos(y)=-2*(sin((x+y)/2)*sin((x-y)/2))

```
using Plots
using test
FX=f(x)=cos(x)
FY=f(y)=cos(y)
g(x,y)=abs(f(x)-f(y))
h(x,y)=-2*(sin((x+y)/2)*sin((x-y)/2))
delta(x,y)=abs(x-y)
plot(FX)
plot!(FY)
plot!(h)
```

@test h=g

plot(FX)

plot!(FY)

plot!(h)