Understanding the Core ConceptHere's what you need to see first: when we say \((t-8)\) is a factor of \(t^2 - kt - 48\), we're actually getting a powerful piece of information. Think about it this way - if \((t-8)\) divides evenly into our polynomial, then when \(t = 8\), the entire polynomial must equal zero. Why? Because \((t-8)\) becomes \((8-8) = 0\), and anything with a factor of zero equals zero.
Let's Apply This InsightNow that we know \(t = 8\) must make the polynomial equal to zero, let's substitute:
When \(t = 8\):
\((8)^2 - k(8) - 48 = 0\)
Working through the arithmetic:
\(64 - 8k - 48 = 0\)
\(16 - 8k = 0\)
Finding the Value of kNotice how this becomes a simple linear equation! Solving for k:
\(16 = 8k\)
\(k = 2\)
So our answer is
(C) 2.
To verify quickly: with \(k = 2\), our polynomial becomes \(t^2 - 2t - 48\). Substituting \(t = 8\): \(64 - 16 - 48 = 0\) ✓
The beauty of this approach is that you don't need to actually factor the polynomial or use polynomial division - you just use the relationship between factors and roots!
Want to Master This Concept?You can check out the
step-by-step solution on Neuron by e-GMAT to understand how this factor-root relationship works across different problem variations and discover time-saving techniques for the GMAT. You can also explore
other GMAT official questions with detailed solutions on Neuron for structured practice with comprehensive analytics into your weaknesses.