(1+tg^2x)*cos(П/2-2x)=-2sqrt{3}
если х=10 то у=6
если х=50 то у=10
если х=120 то у=17
Пускай 2
![x^{2}](https://tex.z-dn.net/?f=+x%5E%7B2%7D+)
+x=y, тогда (у-1)*(у-4)+2=0,
![y^{2}](https://tex.z-dn.net/?f=+y%5E%7B2%7D+)
-5y+4+2=0,
![y^{2}](https://tex.z-dn.net/?f=+y%5E%7B2%7D+)
-5y+6=0, D=25-4*6=1, y1=(5-1)/2=2, y2=(5+1)/2=3. 2
![x^{2}](https://tex.z-dn.net/?f=+x%5E%7B2%7D+)
+x=2, 2
![x^{2}](https://tex.z-dn.net/?f=+x%5E%7B2%7D+)
+x-2=0, D=1-4*2*(-2)=17, x1=(-1-
![\sqrt{17}](https://tex.z-dn.net/?f=+%5Csqrt%7B17%7D+)
)/(2*2)=(-1-
![\sqrt{17}](https://tex.z-dn.net/?f=+%5Csqrt%7B17%7D+)
)/4, x2=(-1+
![\sqrt{17}](https://tex.z-dn.net/?f=+%5Csqrt%7B17%7D+)
)/4. 2
![x^{2}](https://tex.z-dn.net/?f=+x%5E%7B2%7D+)
+x=3, 2
![x^{2}](https://tex.z-dn.net/?f=+x%5E%7B2%7D+)
+x-3=0, D=1-4*2*(-3)=25, x3=(-1-5)/(2*2)=-1,5, x4=(-1+5)/4=1
=-arcsin((√2)/2)+arctg((√3)/3)= -п/4+п/6=-п/12
Х - скорость лодки
4(х+2)+5х=71
4х+8+5х=71
9х+8=71
9х=71-8
9х=63
х=7
<span>Ответ: 7км/ч</span>