Junior high school mathematics substitution video
This limit can be changed from tan5x to sin5x/cos5x, thus becoming 3xcos5x/sin5x. This is a zero-to-zero boundary, which needs to be distinguished up and down. The upper derivative is 3cos5x- 15xsin5x, and the denominator derivative is 5cos5x, so the limit is 3/5 when substituting x=0.