Q: Invert A = [[2,1],[1,1]] by elimination. [A|I] = [[2,1|1,0],[1,1|0,1]]. R₁↔R₂: [[1,1|0,1],[2,1|1,0]]. R₂-2R₁: [[1,1|0,1],[0,-1|1,-2]]. R₂×(-1): [[1,1|0,1],[0,1|-1,2]]. R₁-R₂: [[1,0|1,-1],[0,1|-1,2]]. A⁻¹ = [[1,-1],[-1,2]]! Check: AA⁻¹ = I ✓!