Question

Use the first five terms of the trigonometric series to approximate the value of sin to four decimal places. Then compare
the approximation to the actual value.
a. -0.9568, -0.9659
b. -0.9649, -0.9659
C. 0.5884, 0.5878
d. -0.9609, -0.9659

Answer 1

Quick answer, without using series: sin(4π/5) must be positive since 0 < 4π/5 < π. So C is the only possible choice.

Longer, proper answer:

The first five terms in the series expansion of sin(x) are

sin(x) ≈ x - x ³/3! + x ⁵/5! - x ⁷/7! + x ⁹/9!

Now let x = 4π/5:

sin(4π/5) ≈ 0.5884

so again the answer must be C.

Answer 2

Trigonometric series :  A mathematical series whose terms proceed by sine and cosines of integral multiples of a variable angle.

Option c is correct.

First five term of Trigonometric series of sin is shown below,

   $sin x=x-\frac{x^{3}}{3!}+\frac{x^{5}}{5!}-\frac{x^{7}}{7!}+\frac{x^{9}}{9!}-......$

substitute x = $\frac{4\pi }{5}$

  $sin \frac{4\pi }{5}=\frac{4\pi }{5}-\frac{(\frac{4\pi }{5})^{3}}{3!}+\frac{(\frac{4\pi }{5})^{5}}{5!}-\frac{(\frac{4\pi }{5})^{7}}{7!}+\frac{(\frac{4\pi }{5})^{9}}{9!}-......$

After solving,

We get, approximation value of    $sin\frac{4\pi }{5}=0.5884$

Now find , actual value

              $sin\frac{4\pi }{5}=0.5878$

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