Question

Prove this please

Optional Math




Cos2A =

`(( \cot\alpha - \tan\alpha ) )/(( \cot\alpha + \tan\alpha ) )`

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Answer 1

Explanation:

proof from r.h.s to l.h.s

(cot(a)-tan(a))(cot(a)+tan(a))

cot(a)=cos(a)/sin(a)

tan(a)=sin(a)/cos(a)

(cot(a)-tan(a))=cos(a)/sin(a) - sin(a)/cos(a)

=cos²(a)-sin²(a)/sin(a)cos(a)

from trigonometry identity cos²(a)-sin²(a)=cos2(a)

so we have cos2(a)/sin(a)cos(a)

(cot(a)+tan(a))=cos(a)/sin(a) +sin(a)/cos(a)

=cos²(a)+sin²(a)/cos(a)sin(a)

from trigonometry identity cos²(a)+sin²(a)=so we have 1/cos(a)sin(a)

(cot(a)-tan(a)) ÷(cot(a)+tan(a))

=cos2(a)/cos(a)sin(a) ÷ 1/cos(a)sin(a)

=cos2(a)/cos(a)sin(a) * cos(a)sin(a)

=cos2(a)

proved