A plane flying with a constant speed of 4 km/min passes over a ground radar station at an altitude of 4 km and climbs at an angle of 25 degrees. At what rate, in km/min, is the distance from the plane to the radar station increasing 4 minutes later?
Rate = ????? km/min
Rate = ????? km/min
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Draw a obtuse angled triangle ABC, with angle
B is the position of radar station
A is the position of plane at t = 0
C is the position of plane at t = 4
AB = 4 km
BC is the distance between plane and radar station after 4 min
let BC = s
let AC = = p
dp/dt = 4 km/min
using law of cosines
s^2 = p^2 + 4^2 - 2p*(4)cos(115)
s^2 = p^2 + 16 + 3.384p-------(1)
when t = 4, p = 4*4 = 16 km,
=> s^2 = 256 + 16 + 54.144
==> s^2 = 326.144
s = 18.06 km
now differentiate eqn (1) with respect to t
2s ds/dt = 2p dp/dt + 3.384 dp/dt
s ds/dt = dp/dt (p + 1.692)
substitute s = 18.06, p = 16 and dp/dt = 4
18.06 ds/dt = 4(17.692)
ds/dt = 2*(17.692)/9
= 3.93 km/min
B is the position of radar station
A is the position of plane at t = 0
C is the position of plane at t = 4
AB = 4 km
BC is the distance between plane and radar station after 4 min
let BC = s
let AC = = p
dp/dt = 4 km/min
using law of cosines
s^2 = p^2 + 4^2 - 2p*(4)cos(115)
s^2 = p^2 + 16 + 3.384p-------(1)
when t = 4, p = 4*4 = 16 km,
=> s^2 = 256 + 16 + 54.144
==> s^2 = 326.144
s = 18.06 km
now differentiate eqn (1) with respect to t
2s ds/dt = 2p dp/dt + 3.384 dp/dt
s ds/dt = dp/dt (p + 1.692)
substitute s = 18.06, p = 16 and dp/dt = 4
18.06 ds/dt = 4(17.692)
ds/dt = 2*(17.692)/9
= 3.93 km/min