find the equation of a line perpendicular to the line x-2y+3=0 and passing through the point (1,4)
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find the equation of a line perpendicular to the line x-2y+3=0 and passing through the point (1,4)
find the equation of a line perpendicular to the line x-2y+3=0 and passing through the point (1,4)
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Answer:
we have to find the value of cos20° cos100° + cos100° cos140° - cos140° cos200°
solution : cos20° cos100° + cos100° cos140° - cos140° cos200°
= cos100° [ cos20° + cos140° ] - cos140° cos200°
using formula, cosC + cosD = 2cos(C + D)/2 cos(C - D)/2
= cos100° [2cos80° cos60°] - cos140° cos200°
= cos100° [ 2 cos80° × 1/2 ] - cos140° cos200°
= cos100° cos80° - cos140° cos200°
= 1/2 [ 2cos100° cos80° - 2cos140° cos200°]
using formula,
2cosA cosB = cos(A + B) + cos(A - B)
= 1/2 [ cos180° + cos20° - (cos340° + cos60°)]
= 1/2 [ -1 + cos20° - cos340° - 1/2 ]
= 1/2 [ -3/2 + cos20° - cos(360° - 20°)]
= 1/2 [ -3/2 + cos20° - cos20°]
= 1/2 × (-3/2)
= -3/4
Therefore the value of cos20° cos100° + cos100° cos140° - cos140° cos200° = -3/4