B→C的过程为等压变化,体积增大,为等压膨胀过程. A到B的过程为等容过程,根据查理定律,有: P A T A = P B T B .则 P A P B = T A T B = 300 400 = 3 4 . 根据几何关系, V A V C = OA OC = P A P B = 3 4 对B到C过程,根据盖吕萨克定律有: V B T B = V C T C ,因为V A =V B . 所以 T C T B = V C V B = V C V A = 4 3 , T C = 4 3 T B = 4 3 ×400K 解得T C =533.3K. 故答案为:等压,533.3.