Question

If $15\cot A=8$ ,find the value of sinA and secA.

Answer 1

Hint: In this question, from the given values of cot function by using the trigonometric identities we can find the values of the sin and sec functions. Then on substituting the respective values in the given expression of the question we can calculate the left hand side value and the right hand side value.

Complete step-by-step answer:
Then on comparing the values obtained, we get the result.

& \cos e{{c}^{2}}\theta -{{\cot }^{2}}\theta =1 \\\ & \cos ec\theta =\dfrac{1}{\sin \theta } \\\ & \cos \theta =\cot \theta \cdot \sin \theta \\\ & \sec \theta =\dfrac{1}{\cot \theta \cdot \sin \theta } \\\ \end{aligned}$$ Now, from the given question we have $$\begin{aligned} & 15\cot A=8 \\\ & \cot A=\dfrac{8}{15}\ \ \ \ \ ...(a) \\\ \end{aligned}$$ Now, by using the trigonometric identity which gives the relation between the function that are mentioned in the hint, we get the following substitute the value from the question and as well as from equation (a) $$\begin{aligned} & \Rightarrow \cos e{{c}^{2}}A-{{\cot }^{2}}A=1 \\\ & \Rightarrow \cos e{{c}^{2}}A=1+{{\left( \dfrac{8}{15} \right)}^{2}} \\\ & \Rightarrow \cos e{{c}^{2}}A=1+\dfrac{64}{225} \\\ & \Rightarrow \cos ecA=\sqrt{\dfrac{225+64}{225}}=\sqrt{\dfrac{289}{225}} \\\ & \Rightarrow \cos ecA=\dfrac{17}{15} \\\ \end{aligned}$$ Now, again from the hint, we know that we can get the sin function as follows $$\begin{aligned} & \Rightarrow \cos ecA=\dfrac{1}{\sin A} \\\ & \Rightarrow \sin A=\dfrac{1}{\dfrac{17}{15}} \\\ & \Rightarrow \sin A=\dfrac{15}{17} \\\ \end{aligned}$$ Now, using the relations again, we will get the value of secA as follows $$\begin{aligned} & \Rightarrow \cot A=\dfrac{\cos A}{\sin A} \\\ & \Rightarrow \dfrac{8}{15}=\dfrac{\cos A}{\sin A} \\\ & \Rightarrow \cos A=\dfrac{8}{15}\times \sin A \\\ & \Rightarrow \cos A=\dfrac{8}{15}\times \dfrac{15}{17} \\\ & \Rightarrow \cos A=\dfrac{8}{17} \\\ \end{aligned}$$ Now, for the value of secA as follows $$\sec A=\dfrac{17}{8}$$ Hence, we found the required values as follows $$\begin{aligned} & \sin A=\dfrac{15}{17} \\\ & \sec A=\dfrac{17}{8} \\\ \end{aligned}$$ Note: In such kind of questions, one must be well versed with the relations of trigonometric functions because, without them one could never get to the correct solution. It is important to note that while calculating the values of respective functions we need to use the identities accordingly and solve them. Because neglecting any of the terms or writing it incorrectly changes the complete result.