A long composite string is made up by joining two strings of different mass per unit length  μ  and 4μ respectively. The composite string is under the same tension. A transverse wave pulse Y=6 mmsin5t+40x, where ‘t’ is in seconds and ‘x’ is in metres, is sent along the lighter string towards the joint. The joint is at x=0 . The equation of the wave pulse reflected from the joint is