The best and most correct answer among the choices provided by the question is the fourth choice "d=square root of (x^2+x^1)+(y^2-y^1)^2" The law of cosines<span> for calculating one side of a triangle when the angle opposite and the other two sides are known. Can be used in conjunction with the </span>law<span> of sines to find all sides and angles. </span>
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First the person with a flashlight and one of his friends cross the bridge. After that same person with a flashlight comes back and leads another friend to the other side. Finally he comes back again and leads the last of them. Number of times that he will have to cross the bridge is 5 ( five ).