If cos18^\circ = a /b, then find cosec18° – cos72°.

SSC CPO SI and ASI Model Paper 9

Question : 133 of 200

Marks: +1, -0

cos18° = a /b

, then find cosec18° – cos72°.

Solution:

(2) cosec18°– cos72°
= cosec18°– cos(90° – 18°)
= cosec18° – sin18°

= 1/{sin18}-sin18

={1-sin^218}/{sin18 }

={cos^218}/{{a^2}/{b^2}}/√{1-{a^2}/{b^2}}

{a^2}/{b √{b^2- a^2}}

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