Question

√21cosa, если sina=√5/21,pi/2≤a≤pi  

Answer 1

$cosx = sqrt{1 - sin^{2}x } = -sqrt{1 - frac{5}{21} } = -sqrt frac{16}{21} } = frac{4}{ sqrt{21} }$

$sqrt{21} cosx = -sqrt{21} * frac{4}{ sqrt{21} } = -4$