Question

The length, l mm of an elastic string at time t seconds is given by
l=t^3 / 3 - 4t +10

Find the instants (that is, the value of t ) when
a) The length is increasing at a rate of 5 mm/Sec
b) The length is decreasing at a rate of 4 mm/Sec

Answer 1

l'= t^2-4
5=t^2-4
5+4=t^4
9=t^2
t=root 9
t=3