Solution :
Here, we need to perform JOIN operation of the students and registration.
Use NATURAL JOIN operator 
and so no need to specify Join condition.
Also we need to selection operator tp select only those course whose major in "philosophy".
Finally we projection operator π to project only distinct students name who took at least three courses.
![$\pi [\sigma_{\text{student's nam}e} [ \sigma(\text{three courses}) \Join \text{"Philosophy"}]]$](https://tex.z-dn.net/?f=%24%5Cpi%20%5B%5Csigma_%7B%5Ctext%7Bstudent%27s%20nam%7De%7D%20%5B%20%5Csigma%28%5Ctext%7Bthree%20courses%7D%29%20%5CJoin%20%5Ctext%7B%22Philosophy%22%7D%5D%5D%24)
Change default user name and password is the first step that should be performed by Derek to secure the wireless router
<h3><u>
Explanation:</u></h3>
The device that helps in accessing internet or to any private computer network is the wireless router. It is a devices that acts as the access point which is wireless in nature. There are many steps involved in securing the wireless router. If the wireless router is not secured then it will be used by others without our knowledge.
In order to secure the wireless router firstly we should create our won password and username deleting the default username and password.To do this, we must open router setting page and change the default username and password with our own. The SSID name of the network must also be changed.
Answer:
In Python:
mystr = input("Sentence: ")
mylist = []
word = ""
for i in range(len(mystr)):
if not i == len(mystr)-1:
if not mystr[i] == " ":
word+=mystr[i]
else:
mylist.append(word)
word = ""
else:
word+=mystr[i]
mylist.append(word)
print(mylist)
Explanation:
Prompt the user for an input sentence
mystr = input("Sentence: ")
Initialize an empty list
mylist = []
Initialize each word to an empty string
word = ""
Iterates through the input sentence
for i in range(len(mystr)):
If index is not the last, the following if condition is executed
if not i == len(mystr)-1:
If the current character is not space
if not mystr[i] == " ":
Append the character to word
word+=mystr[i]
If otherwise
else:
Append word to the list
mylist.append(word)
Set word to an empty string
word = ""
If otherwise; i.e. If index is not the last
else:
Append the last character to word
word+=mystr[i]
Append word to the list
mylist.append(word)
Print the list
print(mylist)
Answer:
to create an ArrayList , you declare an Arraylist variable and call the ArrayList constructor to instantiate and ArrayList object and assign it to the variable
