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)
Answer:
QR code
Explanation:
QR Code. Short for quick response code, which can also be called 2-D bar code, a QR code is a type of two-dimensional (2D) barcode that are read with the aid of a QR barcode reader or a camera-enabled smartphone with an inbuilt or installed QR reader software.
A QR code is capable of carrying information in both the vertical and the horizontal direction, which is the reason behind the name 2D barcode.
u have to progrem a math algorithum
It really would depend on what E-Mail system you're using.
If say, you're using G-Mail, the Recipient's address would be placed simply in the "To" bar right above the "Subject" bar.
On Microsoft Word, you can edit your Word.... You can add pictures, do journals, do menus, and stuff like that... You can add color to text, make lists, add shapes to your Word, etc...
