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x^2cos^2α + y^2sin^2α + 2xysinαcosα + x^2sin^2α + y^2c0s^2α - 2xysinαcosα
= x^2(sin^2α + cos^2α) + y^2( sin^2α + cos^2α)
= x^2 + y^2 (terbukti)