If x^2 − 2x − 15 = (x + r)( x + s) for all values of x, and if r and s are constants, then which of the following is a possible value of( r − s)/2?
Select the correct option:  A) 8  B) 2  C) − 2  D) − 4  E) − 5
We know that given ax^2 + bx + c = 0, Sum of the roots = -b/a and product of the roots = c/a.   The roots here are -r and -s.   -r - s = -(-2)/1 = r + s = -2   (-r)*(-s) = -15/1 = rs   So one of r and s is -5 and the other is 3. So (r - s)/2 could be 4 or -4.   Answer (− 4)
<result>D</result>
If x^2 − 2x − 15 = (x + r)( x + s) for all values of x, and if r and s are constants, then which of the following is a possible value of( r − s)/2?