Evaluate-cos-20-cos-40-cos-80-This-question-is-just-for-fun-and-practice-Evryone-who-wants-can-answer-this-question-

Question Number 17983 by alex041103 last updated on 13/Jul/17

E v a l u a t e c o s (20 °) c o s (40 °) c o s (80 °) .

T h i s q u e s t i o n i s j u s t f o r f u n a n d p r a c t i c e .

E v r y o n e w h o w a n t s c a n a n s w e r t h i s q u e s t i o n .

Answered by ajfour last updated on 13/Jul/17

\cos 20 ° \cos 40 ° \cos 80 ° =

\frac{1}{2 \sin 20 °} [2 \sin 20 ° \cos 20 °] \cos 40 ° \cos 80 °

= \frac{1}{4 \sin 20 °} [2 \sin 40 ° \cos 40 °] \cos 80 °

= \frac{1}{8 \sin 20 °} [2 \sin 80 ° \cos 80 °]

= \frac{\sin (180 ° - 20 °)}{8 \sin 20 °} = \frac{1}{8} .

Commented by alex041103 last updated on 13/Jul/17

w e l l d o n e s i r

Commented by alex041103 last updated on 13/Jul/17

i f y o u w a n t y o u c a n t r y Q . 17525