P = cos(80°)∙cos(40°) − sin(80°)∙cos(50°)
= cos(80°)∙cos(40°) − sin(80°)∙cos(90°-40°)
║cos(90°-x) = sin(x)║
P = cos(80°)∙cos(40°) − sin(80°)∙sin(40°)
║cos(x+y) = cos(x)·cos(y) - sin(x)·sin(y)║
P = cos(80° + 40°)
= cos(120°)
= cos(2·60°)
║cos(2x) = cos²(x) - sin²(x)║
P = cos²(60°) - sin²(60)
= (1/2)² - (√3/2)²
= 1/4 - 3/4
= -1/2
