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t in (-oo:+oo)
21/10 = (4/5)*t+(1/2)*t+4/5 // - (4/5)*t+(1/2)*t+4/5
21/10-((4/5)*t)-((1/2)*t)-(4/5) = 0
(-4/5)*t+(-1/2)*t+21/10-4/5 = 0
13/10-13/10*t = 0 // - 13/10
-13/10*t = -13/10 // : -13/10
t = -13/10/(-13/10)
t = 1
t = 1
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