s=20km
v_1
t_1
v_2=v_1-5\frac{km}{h}km
t_2=t_1+20min=t_1+\frac{1}{3}h
s=vt
20=v_1t_1
v_1=\frac{20}{t_1}
20=v_2t_2=(v_1-5)(t_1+\frac{1}{3})
20=(v_1-5)(t_1+\frac{1}{3})
20=(\frac{20}{t_1}-5)(t_1+\frac{1}{3})
20=\frac{20t_1}{t_1}+\frac{20}{3t_1}-5t_1-\frac{5}{3}
20=20+\frac{20}{3t_1}-5t_1-\frac{5}{3}
\frac{20}{3t_1}-5t_1-\frac{5}{3}=0
20-15t_1^2-\frac{5*3t_1}{3}
20-15t_1^2-5t_1=0
-15t_1^2-5t_1+20=0
\Delta=b^2-4ac=(-5)^2-4*(-15)*20=25+1200=1225
\sqrt{\Delta}=\sqrt{1225}=35
t_1=\frac{-b-\sqrt{\Delta}}{2a}=\frac{5-35}{-30}=1
lub
$t_1=\frac{-b+\sqrt{\Delta}}{2a}=\frac{5+35}{-30}=$wynik ujemny
v_1=\frac{20km}{1h}=20\frac{km}{h}
v_2=20\frac{km}{h}-5\frac{km}{h}=15\frac{km}{h}
t_2=1h20min