### Permutation and combination - (14.4-14.5)

#### Understand the concept and notation of combination

• Evaluate each of the following.
a) $$C_1^3 + C_2^3 + C_3^3$$
b) $$C_2^4 \times C_2^5$$
c) $$C_5^9 \div C_7^9$$
• $$\begin{array}{c}a)\qquad C_1^3 + C_2^3 + C_3^3 = \frac{{3{\rm{ }}!}}{{2{\rm{ }}!{\rm{ }} \times 1{\rm{ }}!}} + \frac{{3{\rm{ }}!}}{{1{\rm{ }}!{\rm{ }} \times 2{\rm{ }}!}} + \frac{{3{\rm{ }}!}}{{0!{\rm{ }} \times 3{\rm{ }}!}} \\ = \frac{{3 \times 2}}{{2 \times 1}} + \frac{{3 \times 2}}{{2 \times 1}} + 1\\ = 3 + 3 + 1\\ = \underline{\underline 7} \end{array}$$

$$\begin{array}{c}b)\qquad C_2^4 \times C_2^5 = \frac{{4{\rm{ }}!}}{{2{\rm{ }}!{\rm{ }} \times 2{\rm{ }}!}} \times \frac{{5{\rm{ }}!}}{{3{\rm{ }}!{\rm{ }} \times 2{\rm{ }}!}}\\ = \frac{{4 \times 3}}{{2 \times 1}} \times \frac{{5 \times 4}}{{2 \times 1}}\\ = \underline{\underline {60}} \end{array}$$

$$\begin{array}{c}c)\qquad C_5^9 \div C_7^9 = \frac{{9{\rm{ }}!}}{{4{\rm{ }}!{\rm{ }} \times 5{\rm{ }}!}} \div \frac{{9{\rm{ }}!}}{{2{\rm{ }}!{\rm{ }} \times 7{\rm{ }}!}}\\ = \frac{{9 \times 8 \times 7 \times 6}}{{4 \times 3 \times 2 \times 1}} \times \frac{{2 \times 1}}{{9 \times 8}}\\ = \underline{\underline {\frac{7}{2}}} \end{array}$$

#### Solve problems on the combination of distinct objects without repetition

• a) To choose 4 letters from the word INTERVAL, how many combinations are there?

b) There are 23 members in a soccer team. 11 members are selected among them for
every match. If 4 of the members must be selected and 2 of the members must not
be selected, how many combinations of members are there for the next match?
• a) Number of combinations$$\begin{array}{l} \\= C_4^8\\ = {\underline{\underline {70}} ^{}}\end{array}$$

b) Number of combinations$$\begin{array}{l} \\= C_7^{17}\\ = {\underline{\underline {19{\rm{ }}448}} ^{}}\end{array}$$