V=1,2cos(pi*t)
Eк=mv^2/2=0,7*1,44/2*cos^2(pi*t)=<span>
0,504
</span>*cos^2(pi*t) >= 0,252
0,504
*cos^2(pi*t) >= 0,252
cos^2(pi*t) >= 0,252/<span>0,504=1/2</span>
(cos(2pi*t)+1)/2 >= 1/2
(cos(2pi*t)+1) >= 1
cos(2pi*t) >=0
-pi/2+2*pi*k<= 2pi*t <= pi/2+2*pi*k
-1/4+k<= t <= 1/4+k
при условии 0<=t<=1
получаем 0<= t <= 1/4
ответ t є [0;1/4]