Решить линейное дифиринциальное уравнение.
y'-y*tgx=y^(4)*cosx

I would like to visit the beautiful country italy.i dream to walk through the narrow streets of Rome and enjoy the beauty of the "eternal city." dreaming to go to a pizza and taste the real Italian pizza, which I so often hear.
Also, I would like to visit the Colosseum, Roman Forum, Roman Catholic hrams.my dream also visit the famous beauty fountain De Trevie.Arrive to Rome is complete absorption in the art arhetekturs.big St. Peter's Basilica in the Vatican, the stories of an incredible sight! if I got the opportunity to visit not only consider, I would still like to visit a legendary city Vinece.sometimes, before going to sleep, I imagine I'm floating in a gondola through the channels, and look at the Venetian arhitekturs.next town which I wish to visit it this verona.verona city ​​in the north - east of Italy, which I learned from the story of Romeo and Juliet.i reread several times and it is a sad product of it each time more and more I liked.imbued with spirit of Montague and kapuleti, see the balcony from which Juliet listened to fiery speeches Romeo, there's nothing more beautiful! recently released film "Letters to Juliet."it takes place in Verona, the city finds a letter of the people who was inlove.a woman has left letter to "Juliet" many years ago, and decides to track down the girl (now aged woman in years) and her Romeo . I took not so much plot, exchange captured views of the city verona.so every city in Italy has its own feature, so if any turned up case, I would have traveled throughout Italy, from Rome to the Alps mountains.

Пусть полок х 
тогда
на каждую полку помещается 640:х=640/Х

640/Х+8 =(640+8х)/х это на каждую полку положили 8 книг
(х-4) стало полок

(х-4)(640+8х)/х=640 домножим на х
(х-4)(640+8х)=640х
640х-2560+8х²-32х-640х=0
8х²-32х-2560=0
х²-4х-320=0
Д=16+1280=1296=36²
х1=(4-36)/2=-16 не подходит количество полок больше нуля

х2=(4+36)/2=20  полок

Проверка

1)640:20=32 книги на одной полке
2)32+8=40 книг стало на полке когда положили еще 8
3)20-4=16 столько полок получилось заполненых
4)16*40=640 книг
ответ верный

ответ было 20 полок в стелаже