Find, with proof, the numerical value of (sin 10 degrees)(sin 50 degrees)(sin 70 degrees).
sin (10°) sin (50°) sin(70°) =
sin(70°)sin(50°)sin(10°) = [ because sin A = cos (90 - A) we can write ]
cos(20°) cos(40°)cos(80°)
Note that
sin (2A) = 2 sinA cos A solve for cos A and we have that
cos A = sin 2A
_______
2 sin A
So
cos (20°) = sin (40°)
_________
2 sin (20°)
cos(40°) = sin (80°)
_________
2sin(40°)
cos(80°) = sin (160°)
__________
2sin(80°)
Multiplying these together, we have that
sin (40°) * sin ( 80°) * sin (160°)
_______ _________ __________ =
2sin(20°) 2 sin(40°) 2sin(80°)
sin (160°)
_________ = [ note that sin 20° = sin (180 - 20°) = sin 160°]
8 sin(20°)
1
__
8
BTW......this is the application of something known as " Morrie's Law"
The name is due to the famous physicist Richard Feynman, who used to refer to the identity under that name. Feynman picked that name because he learned it during his childhood from a boy with the name Morrie Jacobs and afterwards remembered it for all of his life.