Angle α is in quadrant III, and angle β is in quadrant II. If sin α = –4∕5 and sin β = 1∕2, find cos (α + β).

Step-by-step explanation:

Given sin α = –4∕5 and sin β = 1∕2

To get α from sin α = –4∕5,

α = arcsin(-4/5)

α = arcsin (-0.8)

α = -53.13°

If angle α is in quadrant III, then α = 180+53.13 = 233.13° (sin is negative in the 3rd quadrant)

Similarly for sin β = 1∕2

β = arcsin(1/2)

β = arcsin(0.5)

β = 30°

Since  β is in quadrant II,  β = 180-30 = 150°

To find  cos (α + β). where α = 233.13° and  β = 30°

cos (α + β)= cos (233.13 + 150)

= cos 383.13°

cos (α + β) = 0.9196

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