Two-element combinations

Write all two-element combinations from elements a, b, c, d.

Correct answer:

n =  6

Step-by-step explanation:

$$\begin{gathered} C_2(4)=\left({\displaystyle \genfrac{}{}{0px}{}{4}{2}}\right)={\displaystyle \frac{4!}{2!(4-2)!}}={\displaystyle \frac{4\cdot 3}{2\cdot 1}}=6 \\ n=\left(\genfrac{}{}{0px}{}{4}{2}\right)=6 \\ ab,ac,ad,bc,bd,cd \end{gathered}$$

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