physics
posted by amy .
Radio waves of frequency 30 kHz are received at a location 1500 km from a transmitter. The radio reception
temporarily “fades” due to destructive interference between the waves travelling parallel to the ground and the
waves reflected from a layer (ionosphere) of the earth’s atmosphere.
(i)What is the minimum distance that the reflected waves should travel for destructive interference to occur
at the receiver?
(ii) The layer at which the waves are reflected is at a height h above the ground. Calculate the minimum
height of this layer for destructive interference to occur at the receiver.

what is 1500+lambda/2 ?
lambda=speedlight/frequency.
for ii0, draw the figure. use right triangle relationships, the hypotensue is 750+lambda/4, the base is 750, so figure h. 
lambda works out to be 10^4m
so 1500+10^4/2=1505km
but i'm still stuck on how this is , why do you add the 1500?
ok i'm using pythagoras theorem but I'ts not working out right for me , thanks