프리드버그 1장 연습문제 풀이 보니


어떤 수 t 가 F 에 속한다는 표현이 있던데..


 Let the four vertices of the parallelogram be A, B, C, D counterclockwise. Say x = AB and y = AD . Then the line joining points B and D should be x + s(y − x), where s is in F. And the line joining points A and C should be t(x + y), where t is in F. To find the intersection of the two lines we should solve s and t such that x + s(y − x) = t(x + y). Hence we have (1 − s − t)x = (t − s)y. But since x and y can not be parallel, we have 1 − s − t = 0 and t − s = 0. So s = t = 1 2 and the midpoint would be the head of the vector 1 2 (x + y) emanating from A and by the previous exercise we know it’s the midpoint of segment AC or segment BD.


저기서 F가 뭔가요 ㅠㅜ


t는 그냥 0과 1 사이의 실수일 거 같은데.. F는 무슨 수죠;;