Try this problem from I.S.I. B.Stat Entrance Objective Problem based on Periodic Function.

Periodic Function (B.Stat Objective Question )


If f(x) = \(a_0+a_1cosx+a_2cos2x+….+a_ncosnx\) where \(a_0,a_1,….,a_n\) are non zero real numbers and \(a_n > |a_0|+|a_1|+….+|a_{n-1}|\), then number of roots of f(x)=0 in \( 0 \leq x \leq 2\pi\), is

  • 0
  • at least 2n
  • 53361
  • 5082

Key Concepts


Periodic

Real Numbers

Inequality

Check the Answer


But try the problem first…

Answer:at least 2n

Source
Suggested Reading

B.Stat Objective Problem 710

Challenges and Thrills of Pre-College Mathematics by University Press

Try with Hints


First hint

f is periodic with period \(2\pi\)

here \(0 <|\displaystyle\sum_{k=0}^{n-1}a_kcos(kx)| \leq \displaystyle\sum_{k=0}^{n-1}|a_k|<a_n, x\in\)set of reals

Second Hint

for points \(x_k=\frac{k\pi}{n}\) \(1 \leq k \leq 2n\)

\(f(x_k)=a_n[cos(k\pi)+\theta] |\theta|<1 for 1 \leq k \leq 2n\)

Final Step

f has at least 2n points in such a period interval where f has alternating sign.

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