Question

Помогите, пожалуйста!!

Answer 1

$sin^2alpha+cos^2alpha=1$

$cosalpha=sqrt{1-sin^2alpha}=sqrt{1-(frac{sqrt{51}}{10})^2}=sqrt{1-frac{51}{100}}=sqrt{frac{49}{100}}=frac{7}{10}=0.7$