error[true_, approximate_]:=approximate-true
relativeError[true_, approximate_]:=error[true, approximate]/true

a=100;
b=101;
ang=Pi/180;
a1 = 100.1;

{a, a1, relativeError[a,a1]*100}

{N[b*Cos[ang]-a], N[b*Cos[ang]-a1],relativeError[b*Cos[ang]-a,b*Cos[ang]-a1]*100}

{N[b*Cos[ang]-a]^2, N[b*Cos[ang]-a1]^2,relativeError[(b*Cos[ang]-a)^2,(b*Cos[ang]-a1)^2]*100}

{N[(b*Cos[ang]-a)^2 +(b*Sin[ang])^2], N[(b*Cos[ang]-a1)^2 +(b*Sin[ang])^2],relativeError[(b*Cos[ang]-a)^2 +(b*Sin[ang])^2,(b*Cos[ang]-a1)^2 +(b*Sin[ang])^2]*100}

{N[Sqrt[(b*Cos[ang]-a)^2 +(b*Sin[ang])^2]], N[Sqrt[(b*Cos[ang]-a1)^2 +(b*Sin[ang])^2]],relativeError[Sqrt[(b*Cos[ang]-a)^2 +(b*Sin[ang])^2],Sqrt[(b*Cos[ang]-a1)^2 +(b*Sin[ang])^2]]*100}