Question

प्रश्न.8. cos(-1080°) का मान ज्ञात कीजिये
अथवा
31T​

Answer 1

$\underline{\textsf{To find:}}$

$\textsf{The value of}$

$\mathsf{cos(-1080^{\circ})}$

$\underline{\textsf{Solution:}}$

$\mathsf{Consider,}$

$\mathsf{cos(-1080^{\circ})}$

$\mathsf{Using,\;\boxed{\bf\;cos(-A)=cosA}}$

$\mathsf{=cos1080^{\circ}}$

$\mathsf{=cos(3(360^{\circ})+0^\circ)}$

$\mathsf{=cos0^\circ}$

$\mathsf{(\because\;\;0^\circ\;and\;1080^\circ\;are\;co-terminus\;angles)}$

$\mathsf{=1}$

$\implies\boxed{\boxed{\bf\mathsf{cos(-1080^{\circ})=1}}}$

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Answer 2

समाधान

निर्धारित करना है

cos(-1080°) का मान

समाधान

Step : 1

Write down the given expression

cos(-1080°)

Step : 2

Cos is an even function

∴ cos(-1080°) = cos 1080°

Step : 3

Express 1080° in terms of 90°

1080° = ( 12 × 90° ) + 0°

Step : 4

The angle 1080° lies in Fourth Quadrant

Step : 5

Now calculate the final value

$\sf{ \cos( - {1080}^{ \circ}) }$

$= \sf{ \cos {1080}^{ \circ} }$

$= \sf{ \cos( 12 \times {90}^{ \circ} + {0}^{ \circ}) }$

$= \sf{ \cos {0}^{ \circ} }$

$= 1$

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