OG详解-OG7 数学1 Q42

Choice B is correct. Since it’s given that P is the center of a circle with a radius of  9 inches, and that points Q and R lie on that circle, it follows that PQ​ and RPRP​ of triangle PQR each have a length of 9 inches. Let the length of  QR​ be x inches. It follows that the perimeter of triangle PQR is 9+9 +x inches. Since it’s given that the perimeter of triangle PQR is 31 inches, it follows that 9+9+x = 31, or 18+x = 31. Subtracting 18 from both sides of this equation gives x = 13. Therefore, the length, in inches, of QR​  is 13.
Choice A is incorrect and may result from conceptual or calculation errors. Choice C is incorrect and may result from conceptual or calculation errors. Choice D is incorrect and may result from conceptual or calculation errors.