Examination

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

Correct answer:

n =  210

Step-by-step explanation:

$n=C_2(21)=\left({\displaystyle \genfrac{}{}{0px}{}{21}{2}}\right)={\displaystyle \frac{21!}{2!(21-2)!}}={\displaystyle \frac{21\cdot 20}{2\cdot 1}}=210$

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