Posted by Solly on .
Cos70cos10+sin70sin10 e =2cos^2 2x1

Maths 
Reiny,
Cos70cos10+sin70sin10 e =2cos^2 2x1
don't know what that "e" is supposed to be, seems to be superfluous.
recall : cos (AB) = cosAcoB + sinAsinB
Cos70cos10+sin70sin10 =2cos^2 2x1
cos(7010) = 2cos^2 2x  1
cos60 = 2cos^2 2x  1
recall cos 2A = 2cos^2 A  1
so 2cos^2 2x  1 = cos 4x
cos 4x = cos60
4x = 60° or 4x = 300°
x = 15° or x = 75°
since cos 2x has a period of 180°
other solutions would be
x = 15+180 = 195°
or
x = 75+180 = 255°