a/b = sinA /sin B = sin2B/sinB = 2sinBcosB/sinB = 2cosBA=2B锐角三角形 任意两个角之和应该大于90°,小于180°90°< A+B = 3B < 180° A = 2B ＜ 90°30°＜ B < 45°√2/2 < cosB < √3/2√2 < 2cosB < √3√2 <...