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If the function $f(x) = {x^3} - 6a{x^2} + 5x$ satisfies the conditions of Lagrange's mean value theorem for the interval $[1, 2] $ and the tangent to the curve $y = f(x) $ at $x = {7 \over 4}$ is parallel to the chord that joins the points of intersection of the curve with the ordinates $x = 1$ and $x = 2$. Then the value of $a$ is
${{35} \over {16}}$
${{35} \over {48}}$
${7 \over {16}}$
${5 \over {16}}$
Solution
(b) $f(b) = f(2) = 8 – 24a + 10 = 18 – 24a$
$f(a) = f(1) = 1 – 6a + 5 = 6 – 6a$
$f'(x) = 3{x^2} – 12ax + 5$
From Lagrange's mean value theorem,
$f'(x) = \frac{{f(b) – f(a)}}{{b – a}}$$ = \frac{{18 – 24a – 6 + 6a}}{{2 – 1}}$
$\therefore f'(x) = 12 – 18a$
At $x = \frac{7}{4},\;3 \times \frac{{49}}{{16}} – 12a \times \frac{7}{4} + 5 = 12 – 18a$
==> $3a = \frac{{147}}{{16}} – 7$
==> $3a = \frac{{35}}{{16}}$ ==> $a = \frac{{35}}{{48}}$.