Answer:

Explanation:
From the question we are told that
Distance b/w A&B 
Frequency 
Sound speed 
Generally the equation for wavelength is mathematically given as




Generally the destructive interference X is mathematically given by


Therefore the destructive interference is

Answer:
4th answer
Explanation:
The gradient of a distance-time graph gives the speed.
gradient = distance / time = speed
Here, the gradient is a constant till 30s. So it has travelled at a constant speed. It means it had not accelarated till 30s. and has stopped moving at 30s.
Answer:
10miles/second
Explanation:
Change in velocity of the bird is expressed as the difference between the final velocity and initial velocity of the body.
Change in velocity = Final velocity - initial velocity
Since the bird takes off from the tree, the initial velocity of the bird = 0miles/sec
Final velocity = 10miles/secs
Change in velocity = 10-0
Change in velocity = 10miles/second
Answer:
The helicopter uses 35 gallons to fly for 5 hours.
Explanation:
The amount of gas that a helicopter uses for flying varies directly proportional to the number of hours spent flying.
g ∝ T
where g represents amount of gas and T time of flight.
Then,

The helicopter files 4 hours and uses 28 gallons of fuel.
Here, g₁= 28 gallons, T₁=4 hours
g₂=?, T₂=5 hours.


⇒28×5= g₂×4
⇒ g₂×4=28×5

gallons
The helicopter uses 35 gallons to fly for 5 hours.