(d) Since, $\frac{f(6) - f(1)}{6 - 1} \geq 2$ $\left\lbrack \because\ \ \ f^{'}(x) = \frac{y_{2} - y_{1}}{x_{2} - x_{1}} \right\rbrack$ $\Rightarrow \ \ \ f(6) - f(1) \geq 10$ $\Rightarrow f(6) + 2 \geq 10$ $\Rightarrow \ \ f(6) \geq 8$
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Ph.D. in Physics with 19+ years of teaching experience.