Explanation
⇒ If x≤0, then x12−x9+x4−x+1>0
⇒ If 0<x≤1, then x12+x4(1−x5)+(1−x)>0
⇒ If x>1, then x9(x3−1)+x(x3−1)+1>0
So the expression x12−x9+x4−x+1>0 is valid for −∞<x<∞
We have to find the velocity along shoreline or parallel to river.
We can see in figure that the velocity V makes an angle (45+15)=600 with the river.
So,
Vcosθ=Vcos600
=18×12
=9km/hr
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