Answer
(a) destructive interference
(b) $3.5m$
Work Step by Step
(a) We know that as the wave reflects from the airplane body, there is a phase change for the wave. As a result, we obtain destructive interference.
(b) We know that
$d_1=\sqrt{(\frac{2l}{3})^2}+h^2$
$d_1=\sqrt{(\frac{2(26Km)}{3})^2+(2.230Km)^2}=24.103Km$
and $d_2=\sqrt{(\frac{l}{3})^2+h^2}=\sqrt{(\frac{36Km}{3})^2+(2.230Km)^2}=12.205Km$
Now $24Km+12Km=x\lambda$
$\implies 36Km=x\lambda$....eq(1)
As the reflected wavelengths are farther away by 88 wavelengths from the direct wave, $24.103Km+12.205Km=(x+88)\lambda$
This simplifies to:
$\lambda=\frac{36.308Km}{x+88}$
We plug in this value in eq(1) to obtain:
$x(\frac{36.308Km}{x+88})=36Km$
This simplifies to:
$\lambda=3.5m$
