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Exercise 2.3 · Q54

Q.If Rolle's theorem holds for the function f(x)=x3+px2+qx+5, x∈[1,3]f(x) = x^3 + px^2 + qx + 5,\ x \in [1, 3] with c=2+13c = 2 + \dfrac{1}{\sqrt3}, find the values of pp and qq.

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f(1)=1+p+q+5=6+p+qf(1)=1+p+q+5=6+p+q. f(3)=27+9p+3q+5=32+9p+3qf(3)=27+9p+3q+5=32+9p+3q.

Rolle's requires f(1)=f(3)f(1)=f(3): 6+p+q=32+9p+3q⇒8p+2q=−26⇒4p+q=−136+p+q=32+9p+3q \Rightarrow 8p+2q=-26 \Rightarrow 4p+q=-13 ... (A)

f′(x)=3x2+2px+qf'(x)=3x^2+2px+q. With c=2+13c=2+\dfrac{1}{\sqrt3}: c2=4+43+13c^2=4+\dfrac{4}{\sqrt3}+\dfrac13, so 3c2=13+433c^2=13+4\sqrt3.

f′(c)=3c2+2pc+q=(13+43)+2p(2+13)+q=13+43+4p+2p3+q=0f'(c)=3c^2+2pc+q = \left(13+4\sqrt3\right) + 2p\left(2+\dfrac{1}{\sqrt3}\right)+q = 13+4\sqrt3+4p+\dfrac{2p}{\sqrt3}+q = 0 …

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