If cos θ = 54 and cos φ = 1312, with θ and φ both in the fourth quadrant, the value of cos(θ + φ) is ?
Explanation
Concept:
cos(A + B) = cos A cos B - sin A sin B
sin²x + cos²x = 1
In the fourth quadrant, the values for cos are positive only.
Calculation:
12esiooo
Here,cosθ=54andcosϕ=1312
sin2θ=1−cos2θ
=1−(54)2
=259
sinθ=± 3/5
=-3/5 .....(∵θ is in the fourth quadrant)
sin2ϕ=1−cos2ϕ
=1−(1312)2
=16925
sinϕ=± 5/13
=-5/13
Now, cos(θ+ϕ)=cosθcosϕ−sinθsinϕ
=54× 1312×−(5−3× 13−5)
=6533
Explanation
Concept:
cos(A + B) = cos A cos B - sin A sin B
sin²x + cos²x = 1
In the fourth quadrant, the values for cos are positive only.
Calculation:
12esiooo
Here,cosθ=54andcosϕ=1312
sin2θ=1−cos2θ
=1−(54)2
=259
sinθ=± 3/5
=-3/5 .....(∵θ is in the fourth quadrant)
sin2ϕ=1−cos2ϕ
=1−(1312)2
=16925
sinϕ=± 5/13
=-5/13
Now, cos(θ+ϕ)=cosθcosϕ−sinθsinϕ
=54× 1312×−(5−3× 13−5)
=6533