Because the coefficients of cosα-2sinα are 1 and 2. Therefore, when the supporting role is matched as a separate cos, it should be matched with a tan value of 2. Let me restore the detailed process for you. At first, I didn't know whether to match a coefficient, so I used X. Later, I found out the matching angle ψ, that is, tan=2, so I could deduce that it was a match √ 5. Then find the minimum value.