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Mathematics

If θ is an acute angle and tan θ = 815\dfrac{8}{15}, find the value of sec θ + cosec θ.

Trigonometric Identities

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Answer

sec2 θ = 1 + tan2 θ

sec2 θ = 1 + (815)2\Big(\dfrac{8}{15}\Big)^2

sec2 θ = 1 + 64225=225+64225=289225\dfrac{64}{225} = \dfrac{225 + 64}{225} = \dfrac{289}{225}.

sec θ = 289225=1715\sqrt{\dfrac{289}{225}} = \dfrac{17}{15}.

cot θ = 1tan θ=1815=158.\dfrac{1}{\text{tan θ}} = \dfrac{1}{\dfrac{8}{15}} = \dfrac{15}{8}.

cosec2 θ = 1 + cot2 θ

cosec2 θ = 1 + (158)2\Big(\dfrac{15}{8}\Big)^2

cosec2 θ = 1 + 22564=64+22564=28964\dfrac{225}{64} = \dfrac{64 + 225}{64} = \dfrac{289}{64}.

cosec θ = 28964=178\sqrt{\dfrac{289}{64}} = \dfrac{17}{8}.

sec θ + cosec θ=1715+178=17×8+17×15120=136+255120=391120=331120.\text{sec θ + cosec θ} = \dfrac{17}{15} + \dfrac{17}{8} \\[1em] = \dfrac{17 \times 8 + 17 \times 15}{120} \\[1em] = \dfrac{136 + 255}{120} \\[1em] = \dfrac{391}{120} \\[1em] = 3\dfrac{31}{120}.

Hence, the value of expression sec θ + cosec θ = 331120.3\dfrac{31}{120}.

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