Solve for c:
921600 = 518400+c^2-1440 c cos(20°)
921600 = 518400+c^2-1440 c cos(20°) is equivalent to 518400+c^2-1440 c cos(20°) = 921600:
518400+c^2-1440 c cos(20°) = 921600
Subtract 518400 from both sides:
c^2-1440 c cos(20°) = 403200
Add 518400 cos(20°)^2 to both sides:
c^2-1440 c cos(20°)+518400 cos(20°)^2 = 518400 cos(20°)^2+403200
Write the left hand side as a square:
(c-720 cos(20°))^2 = 518400 cos(20°)^2+403200
Take the square root of both sides:
c-720 cos(20°) = sqrt(518400 cos(20°)^2+403200) or c-720 cos(20°) = -sqrt(518400 cos(20°)^2+403200)
Add 720 cos(20°) to both sides:
c = sqrt(518400 cos(20°)^2+403200)+720 cos(20°) or c-720 cos(20°) = -sqrt(518400 cos(20°)^2+403200)
Add 720 cos(20°) to both sides:
Answer: | c = sqrt(518400 cos(20°)^2+403200)+720 cos(20°) or c = 720 cos(20°)-sqrt(518400 cos(20°)^2+403200)