- #1
disfused_3289
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Homework Statement
Given (d^2)x/ (dt^2) = 2 for all t>= 0 and dx/dt= -5 and x= 4 when t= 0 find t when
a) dx/ dy= 0
b) x= 0
2. The attempt at a solution
a) Given (d^2)x/ (dt^2) = 2, then
dx/dt= 2x + c = -5
given dx/ dt= 0 then 2x + c= 0
c= -2.5 using 2x + c= 0 and 2x + c = -5
using dx/dt= 2x + c, t= x^2 + cx + d
where 0= 16 - 2.5 x 4 + d (using x= 4 when t= 0 )
where d= -6
Thus,
t= x^2 - 2.5x - 6
when dx/dt= 0 then x= 1.25 in 2x + c= 0 (c= -2.5)
subsituting x= 1.25 into x^2 - 2.5x - 6 gives t= -121/16,
however the answer seems to differ giving t= 2.5 when dx/dt= 0
How is this so?
b) not to sure as confused about a).