Решение 


Замена:  sin x = t  ⇒


x = `+`(`*`(`/`(1, 6), `*`(`^`(-1, `+`(k, 1)), `*`(Pi))), `*`(Pi, `*`(k))), `in`(k, N) 

Проверка 

> solve(`+`(`*`(2, `*`(`^`(t, 2))), `-`(`*`(5, `*`(t))), `-`(3)) = 0); 1
 

3, -`/`(1, 2) (21.2.1)
 

> restart; 1; with(RealDomain); -1; solve(`+`(`*`(2, `*`(RealDomain:-`^`(sin(x), 2))), `-`(`*`(5, `*`(sin(x)))), `-`(3)) = 0); 1
 

`+`(`-`(`*`(`/`(1, 6), `*`(Pi)))) (21.2.2)