Answer
See answer below
Work Step by Step
We are given $S=\{(1,2,-1,0),(1,0,1,2),(-1,1,1,0),(1,-1,-1,0)\}$ in $R^4$
We will check: $(v_1,v_2)==(-1).1+1.(-1)+1.(-1)_0=-3 \ne 0$
Hence, $S$ is not an orthogonal set in $R^4$
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