This ques can be done by plugging numbersHG10 wrote:If (r - 4) is a factor of r^2 - kr - 36, then k =
(a) 16
(b) 5
(c) 0
(d) - 5
(e) - 16
Answer D
say r = 7
3 is a factor of 49-7k-36
13-7k
now plug each value and eliminate answer choices
(A)16
13 - 112 = 99
3 is a factor of 99
Keep it
(b)5
13-35 = -22
3 is not a factor of -22
Eliminate
(c)0
13-0
3 is not a factor of 13
Eliminate
(D)-5
13+35
48
3 is a factor of 48
Keep it
(E)-16
13+112
125
3 is not a factor of 125
Eliminate
Answer choices left A and D
Now put r= 9
(9-4) is a factor of 81-9k-36
5 is a factor of 45-9k
(A)16
45-9(16) = not divisible by 5
Eliminate
Only D is left so you can mark it and move on.
or you can check, if you want to waste time
Option D
