arcsin(-1/sqrt(2)) = alphaarcsin(−1√2)=α where alpha in [-pi/2, pi/2]α∈[−π2,π2] such that sin(alpha) = -1/sqrt(2)sin(α)=−1√2
1/sqrt(2)1√2 is one side of the right angled triangle with sides:
1/sqrt(2)1√2, 1/sqrt(2)1√2 and 11
(Note (1/sqrt(2))^2 + (1/sqrt(2))^2 = 1/2 + 1/2 = 1 = 1^2(1√2)2+(1√2)2=12+12=1=12)
This triangle has angles pi/4π4, pi/4π4 and pi/2π2.
So sin(pi/4) = 1/sqrt(2)sin(π4)=1√2
Now sin(-theta) = -sin(theta)sin(−θ)=−sin(θ)
So sin(-pi/4) = -1/sqrt(2)sin(−π4)=−1√2
So arcsin(-1/sqrt(2)) = -pi/4arcsin(−1√2)=−π4