Is it possible to include numerical values or constants within brackets(time complexity)?
What you read is slightly unusual, but absolutely correct. Follow the definition of Big-O literally: It means there is a c > 0 such that for large n, f(n) &lt;= c * 0.5 * n^2. It’s exactly the same as O(n^2), except that the constant c is different (twice as big will work). 

In your example, for n >= 5 we have 5n &lt;= n^2 and therefore 5n^2 + 5n &lt;= 6n^2. So f(n) &lt;= 12 * 0.5 n^2 and f(n) &lt;= 6*n^2, c=12 instead of c=6.