Question:

<p>Treating the equation as a quadratic in <span class="math">\(c^2\)</span>: <span class="math">\(c^4 - 2(a^2 + b^2)c^2 + a^4 + a^2b^2 + b^4 = 0\)</span></p><p>Using the quadratic formula or factoring: <span class="math">\(c^2 = a^2 + b^2 - ab\)</span></p><p>By the cosine rule: <span class="math">\(c^2 = a^2 + b^2 - 2ab\cos C\)</span></p><p>Comparing: <span class="math">\(-2ab\cos C = -ab\)</span>, so <span class="math">\(\cos C = \frac{1}{2}\)</span>, hence <span class="math">\(C = 60°\)</span></p><p>∴ Answer is (a) <span class="math">\(60°\)</span>.</p>