Examination

The class is 21 students. How many ways can choose two to examination?

Correct result:

n =  210

Solution:

n=C2(21)=(212)=21!2!(21−2)!=21⋅202⋅1=210n = C_{{ 2}}(21) = \dbinom{ 21}{ 2} = \dfrac{ 21! }{ 2!(21-2)!} = \dfrac{ 21 \cdot 20 } { 2 \cdot 1 } = 210

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