Trigonometry 数学题

kuboy1314

请问 a 和 c 怎么解？

57773

(c)(i)
cos 2x=(cos (x))^2-(sin( x))^2
(cos (x))^2-(sin( x))^2=0.5
Since (sin( x))^2 +(cos (x))^2=1
2*(cos (x))^2=3/2
cos(x)=sqrt(3/4), -sqrt(3/4)
However the question request obtuse angle,
So, cos(x)=-sqrt(3/4)

(c)(ii)
2*(sin (x))^2=0.5
sin(x)=sqrt(1/4), -sqrt(1/4)
However the question request obtuse angle,
So, sin(x)=sqrt(1/4)