# Homework Help: Spontaneous disintegration in classical mechanics

1. Jan 21, 2012

### ilp89

Could someone demonstrate to me how in Landau's Mechanics book, he gets from equation (16.5)

tan θ = (v_0 sin θ_0) / (v_0 cos θ_0 + V)

to equation (16.6)

cos θ_0 = -(V/v_0) sin^2 θ ± cos θ √[1 - (V/v_0)^2 sin^2 θ]

I am using the quadratic formula, and the first term on the right comes out fine, but I can't get the second term.

Thanks!

2. Jan 25, 2012

### vela

Staff Emeritus
It worked out for me. What did you get for the quadratic you needed to solve and what did you get for the discriminant?

3. Jan 25, 2012

### genericusrnme

Oh boy, I remember doing that one
a lot of the equations in those books are quite annoying (not 'hard' exactly) to arrive at.
you just need to make use of $sin=\sqrt{1-cos^2}$

there are a lot more tricky ones later on though so good luck