SAT Math: Algebra


If one solution of the equation x2 + kx + 15 = 0 is -5, what is the value of k ?

A.   -8

B.   -4

C.   2

D.   4

E.   8


Knowsys Method

Read the question carefully.  To solve this problem, you must know the definition of a solution.  

Identify the bottom line.  Make a note: k = ?

Assess your options.  You could plug in answer choices, but it may take a lot of time to eliminate incorrect answers.  Instead, think about how you can use the information that you were given.  What variable are you normally solving for in these equations?  An x!  The problem tells you that one of the solutions is -5.  That means that x can equal -5.  Once you plug in x = -5, it will be easy to solve for k.

Attack the problem.


x2 + kx + 15 = 0
(-5)2 + k(-5) + 15 = 0
25 - 5k + 15 = 0
-5k = -40
k = 8

Loop back.   You solved for k, which is your bottom line.  Look down at your answer choices.


The correct answer is (E). 

This is a medium level question.


Want some help reviewing the math concepts you need to master?  Try out the Knowsys Pre-Algebra Flashcards, the Knowsys Algebra I Flashcards, and the Knowsys SAT & ACT Math Practice book.  

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