Решить уравнение  3sin²x+7cosx-3=0

3(sinx)^2 +7cosx-3=0

3(1-(cosx)^2)+7cosx-3=0

3-3(cosx)^2 +7cosx-3=0

3(cosx)^2-7cosx=0

cosx(3cosx-7)=0

cosx=0                                   или                       3cosx-7=0

x=П/2+Пn, n принадлежит Z                                3cosx=7

                                                                            cosx=7/3

                                                                             корней нет, т.к.

                                                                            |cosx|<=1