2πr₂ (3√2 + r₂) = 12π
3√2 r₂ + r₂² - 6 = 0
r₂² + 3√2 r₂ - 6 = 0
1r₂² + 3√2 – 6 = 0
a = 1, b = 3√2, c = -6
r1.2 = -b ±√b² - 4ac
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2a
= -3√2 ± √(3√2² - 4.1.(-6)
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2.1
= -3√2 ± √18 + 24
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2
= -3√2 ± √42
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2