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### Trigonometric identities - 9

Trigonometric identity:  sin θ / (1 - cos θ) = cosec θ + cot θ

The LHS of this identity does not seem to accept any trigonometric identities that we know, like sin2θ + cos2θ = 1, or tan θ = sin θ / cos θ. One way to solve trigonometric expressions containing rational expressions is to multiply the rational expression with the conjugate of the denominator.

The denominator of the LHS is (1 - cos θ), and its conjugate is (1 + cos θ). Multiplying by (1 + cos θ) / (1 + cos θ) is same as multiplying by 1, which makes no difference to an expression. So multiplying the LHS with (1 + cos θ),
sin θ / (1 - cos θ) * [(1 + cos θ) / (1 + cos θ)]
= [ sin θ (1 + cos θ) ] / [ (1 - cos θ) (1 + cos θ) ]
The denominator is in the form of (a - b)(a + b), which can be simplified to a2 - b2,
= [ sin θ (1 + cos θ) ] / [ 1 - cos2θ ]
From the identity sin2θ + cos2θ = 1, it follows that sin2θ = 1 - cos2θ, putting this in the above expression,
= [ sin θ (1 + cos θ) ] / [ sin2θ ]
Cancelling the common sin θ from the numerator and denominator,
=  [ (1 + cos θ) ] / [ sin θ ]
Distributing the denominator sin &theta in the numerator,
= 1/sin θ + cos θ / sin θ
Recall cosec θ = 1/sin θ and cot θ = cos θ / sin θ,
=  cosec θ + cot θ
... which is the RHS expression

Trigonometric identities applied: