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If ∑ = + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − 10 0 r r 1 r 10 1 10 · 11 C r+1 = 10 11 11 10 11 − α , then α is equal to :

1. 20
2. 24
3. 15
4. 11

Correct answer: 20

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