Hi ,
Here I am using A instead of theta .
It is given that ,
Sec A + tan A = k ---( 1 )
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By Trigonometric identity :
Sec²A - tan² A = 1
Algebraic identity :
a² - b² = ( a + b )( a - b )
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Sec² A - tan² A = 1
( SecA + tan A )( secA - tanA ) = 1
k × ( sec A - tan A ) = 1 [ from ( 1 ) ]
Sec A - tan A = 1/k
I hope this helps you.
: )