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If X = \(\frac{3}{5}\) and cos y = \(\frac{24}{25}\), where X and Y are acute,...

Further Mathematics
WAEC 2020

If X = \(\frac{3}{5}\) and cos y = \(\frac{24}{25}\), where X and Y are acute, find the value of cos (X + Y). 

  • A. \(\frac{117}{125}\)
  • B. \(\frac{24}{25}\)
  • C. \(\frac{3}{5}\)
  • D. \(\frac{7}{25}\)
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Correct Answer: Option C
Explanation

Cos (x + y) 

= cos x cos y - sin x sin y

cos (x + y) = \(\frac{4}{5} \times \frac{24}{25} - \frac{3}{5} \times \frac{7}{25}\)

= \(\frac{96}{125} - \frac{21}{125} = \frac{96 - 21}{125}\)

= \(\frac{75}{125}\)

= \(\frac{3}{5}\) 


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