2.設A(-2,a)、B(3,4)、C(2,8)、D(4,-2),若
【用戶】Jiunndar Yeh
【年級】幼兒園下
【評論內容】因AB與CD垂直,故可知兩線之斜率相乘等於-1(即mAB * mCD = -1)mAB = 4 - a/3- (-2) = 4 - a/5mCD = (-2) - 8/4 - (-2) = (-10)/6 = -5/3⇒ 4 - a/5 * (-5/3) = -1 (4 - a) * (-5)/5 * 3 = -1 -20 + 5a = -1 * 15 = -15 5a = -15 + 20 = 5 a = 5/5 = 1 ...選項(B)正確