A long string is lying along X-axis and a tension is maintained in it. Linear mass density is gradually increasing along positive X-axis. A transverse wave is propagating along the string in positive X direction. Then

velocity of propagation, V=TμAs μ is increasing with increase in x, V is decreasing with increase in x. Also since frequency ν remains unchanged during propagation of wave and λ=Vν, wavelength λ decreases with increase in X.