From △OAP, we get that cos(∠OPA)=AP/OP=10/(5√5)=2/√5 But ∠OPA=x/2 i.e., cos(x/2)=2/√5 From cos(x/2), we can find the value of cos(x) using the double-angle formula, namely: cos(2θ)=2cos²(θ)-1 Thus, cos(2*x/2)=2cos²(x/2)-1=2(2/√5)²-1=8/5-1=3/5 i.e., cos(x)=3/5
