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Sati [7]
2 years ago
12

Prove that:cos20°cos40°cos80°=1/8​

Mathematics
2 answers:
Naya [18.7K]2 years ago
7 0

Answer:

see explanation

Step-by-step explanation:

Using the double angle identity for sine

sin2x = 2sinxcosx

Consider left side

cos20°cos40°cos80°

= \frac{1}{2sin20} (2sin20°cos20°)cos40°cos80°

= \frac{1}{4sin20} (2sin40°cos40°)cos80°

= \frac{1}{4sin20} (sin80°cos80° )

= \frac{1}{8sin20} (2sin80°cos80° )

= \frac{1}{8sin20} . sin160°

= \frac{1}{8sin20} . sin(180 - 20)°

= \frac{1}{8sin20} . sin20°

= \frac{1}{8} = right side , thus proven

viva [34]2 years ago
7 0

Step-by-step explanation:

hope its helps you have a great day keep smiling be happy stay safe ☺

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The total area of the complete lawn is (100-ft x 200-ft) = 20,000 ft².
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When you think about it ... massage it and roll it around in your
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First lap:
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       (100 - 2) = 98
       (200 - 2) = 198
       (100 - 4) = 96    

Second lap:
       (200 - 4) = 196
       (100 - 6) = 94
       (200 - 6) = 194
       (100 - 8) = 92   

Third lap:
       (200 - 8) = 192
       (100 - 10) = 90
       (200 - 10) = 190
       (100 - 12) = 88 

These are the lengths of each strip.  They're 2-ft wide, so the area
of each one is (2 x the length). 

I expected to be able to see a pattern developing, but my brain cells
are too fatigued and I don't see it.  So I'll just keep going for another
lap, then add up all the areas and see how close he is:

Fourth lap:
       (200 - 12) = 188
       (100 - 14) = 86
       (200 - 14) = 186
       (100 - 16) = 84 

So far, after four laps around the yard, the 16 lengths add up to
2,272-ft, for a total area of 4,544-ft².  If I kept this up, I'd need to do
at least four more laps ... probably more, because they're getting smaller
all the time, so each lap contributes less area than the last one did.

Hey ! Maybe that's the key to the approximate pattern !

Each lap around the yard mows a 2-ft strip along the length ... twice ...
and a 2-ft strip along the width ... twice.  (Approximately.)  So the area
that gets mowed around each lap is (2-ft) x (the perimeter of the rectangle),
(approximately), and then the NEXT lap is a rectangle with 4-ft less length
and 4-ft less width.

So now we have rectangles measuring

         (200 x 100),  (196 x 96),  (192 x 92),  (188 x 88),  (184 x 84) ... etc.

and the areas of their rectangular strips are
           1200-ft², 1168-ft², 1136-ft², 1104-ft², 1072-ft² ... etc.

==> I see that the areas are decreasing by 32-ft² each lap.
       So the next few laps are 
               1040-ft², 1008-ft², 976-ft², 944-ft², 912-ft² ... etc. 

How much area do we have now:

             After 9 laps,    Area =   9,648-ft²
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And there you are ... Somewhere during the 10th lap, he'll need to
stop and call the company surveyor, to come out, measure up, walk
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There must still be an easier way to do it.  For now, however, I'll leave it
there, and go with my answer of:  During the 10th lap.

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