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## Homework Equations

The equation for a harmonic wave is

r = asin(kx-vt+θ)

a being the amplitude

k being the wave number k=2π/λ

v being the velocity of the wave

θ being the initial phase angle

## The Attempt at a Solution

I set up the wave equations at both positions because we have two unknowns so we need two equations

13 = asin(θ) & -7.5= asin((2π/λ)(3λ/4) + θ)

13/sin(θ ) = a -7.5=asin(3π/2 + θ)

Now I plugged in 13/sin(θ ) for a in the other equation and I ended up with

-7.5=(13/sin(θ ))sin(3π/2 + θ)

-7.5sin(θ ) = 13sin(3π/2 + θ)

This is where I got stuck. Am I on the right track? I imagine there is a trig identity that will help me solve for θ and then I can easily solve for the amplitude. The answer according to the book is:

15sin(kx+π/3)