Sina=4/5; 90 <a <180; sin2a-? sin(a/2)-?

Question

Sina=4/5; 90 <a <180; sin2a-? sin(a/2)-?

Answer 1

Sina=4/5;90°180°<2a<360°<br>cos²a=1-sin²a=1-16/25=(25-16)/25=9/25
cosa=-√(9/25)=-3/5
sin2a=2•sina•cosa=2•4/5•(-3/5)==-24/25
sin(a/2)=?
sin²(a/2)=(1-cosa)/2=(1+3/5)/2=8/10=4/5
90°45°sin(a/2)=+√(4/5)=2/√5=2√5/5