In other words, if you solve a triangle with sine formula, you will find a sine value. Because the sine function is symmetrical in one or two quadrants, there will be two angles. For example, if you find a sine value of 1/2, it may be 30 degrees or 150 degrees ... then let's discuss it and see if the sum of the three internal angles exceeds 180 degrees. Or you can't rule out the specific situation in the topic, or you can't rule out two solutions ... Generally, you don't want to discuss it, just use the cosine theorem directly, with the positive in the quadrant and the negative in the quadrant, but it's very troublesome. ...