rt，为什么 ∣a1−a2∣+∣a1−a3∣+∣a1−a4∣+∣a2−a3∣+∣a2−a4∣+∣a3−a4∣=1×a2−a1+2×a3−a1−a2+3×a4−a1−a2−a3|a_1 - a_2| + |a_1 - a_3| + |a_1 - a_4| + |a_2 - a_3| + |a_2 - a_4| + |a_3 - a_4| = 1\times a_2 - a_1 + 2\times a_3 - a_1 - a_2 + 3\times a_4 - a_1 - a_2 - a_3∣a1−a2∣+∣a1−a3∣+∣a1−a4∣+∣a2−a3∣+∣a2−a4∣+∣a3−a4∣=1×a2−a1+2×a3−a1−a2+3×a4−a1−a2−a3？
