# Given theta is an acute angle such that sin theta = 5/13 find the value of tan (theta - pi4)

Question
Given theta is an acute angle such that sin theta = $$\displaystyle\frac{{5}}{{13}}$$ find the value of tan (theta - pi\4)

2020-12-28
$$\displaystyle{\tan{{\left(\theta-\frac{\pi}{{4}}\right)}}}={\left({\tan{\theta}}-\frac{{\tan{{\left(\frac{\pi}{{4}}\right)}}}}{{{1}+{\tan{\theta}}\frac{{\tan{\pi}}}{{4}}}}=\right.}$$
$$\displaystyle\frac{{{\tan{\theta}}-{1}}}{{{1}+{\tan{\theta}}}}.$$
In a right triangle with hypotenuse length 13 and a leg length of 5, the other leg has a length $$\displaystyle\sqrt{{{13}^{{2}}-{5}^{{2}}}}=\sqrt{{144}}={12}$$. In this triangle, when $$\displaystyle{\sin{\theta}}=\frac{{5}}{{13}}$$ (opp/hyp) then $$\displaystyle{\tan{\theta}}={o}{p}\frac{{p}}{{a}}{d}{j}=\frac{{5}}{{12}}$$.
Therefore $$\displaystyle{\tan{{\left(\theta-\frac{\pi}{{4}}\right)}}}=\frac{{\frac{{5}}{{12}}-{1}}}{{{1}+\frac{{5}}{{12}}}}=\frac{{-\frac{{7}}{{12}}}}{{\frac{{17}}{{12}}}}=-\frac{{7}}{{17}}.$$

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