Question

If $\sec a + \tan a = p$, then show that $\sec a - \tan a = \dfrac{1}{p}$ . Hence find the value of $\cos a$ and $\sin a$.

Answer 1

Hint- For solving this problem use the basic identities of trigonometry such as ${\sec ^2}\theta - {\tan ^2}\theta = 1$ and ${\sin ^2}\theta + {\cos ^2}\theta = 1$.

Given that:
$ \Rightarrow \sec a + \tan a = p$…………………………….. (1)
As we know that
$
  {a^2} - {b^2} = (a + b)(a - b) \\
  {\sec ^2}a - {\tan ^2}a = 1 \\
 $
Using above formula
$
   \Rightarrow {\sec ^2}a - {\tan ^2}a = 1 \\
   \Rightarrow (\sec a + \tan a)(\sec a - \tan a) = 1 \\
 $
Using the value given in above equation, we get
$
   \Rightarrow p(\sec a - \tan a) = 1 \\
   \Rightarrow \sec a - \tan a = \dfrac{1}{p} \\
$………………………………… (2)
Hence, we have arrived at our first result.
Now, we have to find out the value of $\cos a$ and $\sin a$.
By adding equation (1) and (2) and further solving , we obtain
$
  (\sec a + \tan a) + (\sec a - \tan a) = p + \dfrac{1}{p} \\
  2\sec a = p + \dfrac{1}{p} \\
  \sec a = \dfrac{{p + \dfrac{1}{p}}}{2} \\
$
As we know
 \[\because \cos \theta = \dfrac{1}{{\sec \theta }}\]
$
  \cos a = \dfrac{2}{{p + \dfrac{1}{p}}} \\
  \cos a = \dfrac{{2p}}{{{p^2} + 1}} \\
$
As we know
 $
  \because {\sin ^2}a + {\cos ^2}a = 1 \\
   \Rightarrow \sin a = \sqrt {1 - {{\cos }^2}a} \\
 $
Using the value of $\cos a$ in above equation and further solving it, we get
$
  \sin a = \sqrt {1 - \dfrac{{4{p^2}}}{{1 + 2{p^2} + {p^4}}}} \\
   = \sqrt {\dfrac{{1 + 2{p^2} + {p^4} - 4{p^2}}}{{1 + 2{p^2} + {p^4}}}} \\
   = \sqrt {\dfrac{{1 - 2{p^2} + {p^4}}}{{1 + 2{p^2} + {p^4}}}} \\
   = \sqrt {\dfrac{{{{(1 - {p^2})}^2}}}{{{{(1 + {p^2})}^2}}}} {\text{ }}\left[ {\because {{\left( {a + b} \right)}^2} = {a^2} + {b^2} + 2ab\& {{\left( {a - b} \right)}^2} = {a^2} + {b^2} - 2ab} \right] \\
  \sin a = \dfrac{{(1 - {p^2})}}{{(1 + {p^2})}} \\
$
So, the values of $\cos a = \dfrac{{2p}}{{1 + {p^2}}}$ and $\sin a = \dfrac{{(1 - {p^2})}}{{(1 + {p^2})}}$.

Note- Before solving these types of problems you must remember all the trigonometric identities and try to bring all the terms in a single variable. All the same terms will cancel out.