a) v²=v₀²-2*g*hmax<=>0²=40²-20hmax<=>20hmax=1600=>hmax=1600/20=80 m
v=v₀-gt<=>0=40-10t=>t=40/10=4 s
b) h=3hmax/4=3*80/4=240/4=60 m
x₁=v₀t'-gt'²/2
x₂=h-gt'²/2
x₁=x₂=>v₀t'-v₀₂t-gt'²/2=h-gt'²/2=>v₀t'=h=>t'=h/v₀=60/40=1,5 s
hi=h-gt'²/2=60-10*1,5²/2=60-5*2,25=60-11,25=48,75 m
c) v₁=v₀-gt'=40-10*1,5=40-15=25 m/s
v₂=v₀₂-gt'=-10*1,5=-15 m/s
vr=v₁-v₂=25+15=40 m/s