OG详解-OG1 数学2 Q5

Choice D is correct. By the Pythagorean theorem, if a right triangle has a hypotenuse with length c and legs with lengths a and b, then c²= a² +b². In the right triangle shown, the hypotenuse has length c and the legs have lengths  a and b. It’s given that a = 4 and b = 5. Substituting 4 for a and 5 for b in the Pythagorean theorem yields c²= 4² +5² .Taking the square root of both sides of this equation yields   . Since the length of a side of a triangle must be 
positive, the value of c is .