COMPLEMENTARY ANGLES IN TRIGONOMETRY

About "Complementary angles in trigonometry"

Complementary angles in trigonometry :

Two acute angles are complementary to each other if their sum is equal to 90°. In a right triangle the sum of the two acute angles is equal to 90°. So, the two acute angles of a right triangle are always complementary to each other.

Let ABC be a right triangle, right angled at B.  

If <ACB = θ, then <BAC = 90° - θ and hence the angles <BAC and <ACB are complementary

For the angle θ, we have 

Similarly, for the angle (90° - θ), we have

Comparing the equations in (1) and (2) we get,

Key concept      -     Trigonometric Ratios of Complementary Angles

Complementary angles in trigonometry - Practice problems

Problem 1 :

Evaluate  :  cos 56° / sin 34°

Solution : 

The angles 56° and 34° are complementary.

So, using trigonometric ratios of complementary angles, we have

cos 56°  =  cos (90° - 56°)  =  sin 34°

cos 56° / sin 34°  =  sin 34° / sin 34°  =  1   

Hence the value of cos 56° / sin 34° is 1.

Problem 2 :

Evaluate  :  tan 25° / cot 65°

Solution : 

The angles 25° and 65° are complementary.

So, using trigonometric ratios of complementary angles, we have

tan 25°  =  tan (90° - 65°)  =  cot 65°

tan 25° / cot 65°  =  cot 65° / cot 65°  =  1   

Hence the value of tan 25° / cot 65° is 1.

Problem 3 :

Evaluate  :  (cos 65° sin 18° cos 58°) / (cos 72° sin 25° sin 32°)

Solution : 

Using trigonometric ratios of complementary-angles, we have

cos 65°  =  cos (90° - 25°)  =  sin 25°

sin 18°  =  sin (90° - 72°)  =  cos 72°

cos 58°  =  cos (90° - 32°)  =  sin 32°

(cos 65° sin 18° cos 58°) / (cos 72° sin 25° sin 32°) is

=  (sin 25° cos 72° sin 32°) / (cos 72° sin 25° sin 32°)

=  1

Hence the value of the given trigonometric expression is 1.

Problem 4 :

Prove  :  tan 35° tan 60° tan 55° tan 30°   =  1

Solution : 

Using trigonometric ratios of complementary-angles, we have

tan 35°  =  tan (90° - 55°)  =  cot 55°  =  1/tan 55°

tan 60°  =  tan (90° - 30°)  =  cot 30°  =  1/tan 30°

tan 35° tan 60° tan 55° tan 30°  is 

=  (1/tan 55° ) x (1/tan 30°) tan 55° tan 30° 

=  1

Hence, tan 35° tan 60° tan 55° tan 30°   =  1

Problem 5 :

If sin A  =  cos 33°, find A

Solution : 

Using trigonometric ratios of complementary-angles, we have

sin A  =  cos (90° - A) 

Therefore,

sin A  =  cos 33° -----> cos (90° - A)  =  cos 33° 

  90° - A  =  33°

90° - 33°  =  A

57°  =  A

Hence, A is 57°.

After having gone through the stuff given above, we hope that the students would have understood "Complementary angles in trigonometry"

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