The function f,defined by f(t)=6+cos(t/40)+3sin(7t/40),is used to model the velocity of the plane,in miles per minute,for 0≤t≤40.According to thhis model,waht is the acceleration of the plane at t=23?What is the average velocity of the plane,in miles per minute,over the time interval 0≤t≤40?