На промежутке [2п;7п/2]
cos^2x/sin^2x + 3/sinx + 3 = 0
все умножаем на sin^2x
cos^2x + 3sinx + 3sin^2x = 0
1-sin^2x + 3sinx + 3sin^2x = 0
2sin^2x + 3sinx + 1 = 0
sinx = t
2t^2+3t+1 = 0
t=-1 и t=-1/2
sinx = -1
x = -Pi/2+2Pik
sinx=-1/2
x = (-1)^(k)(-Pi/6)+Pik