# How to Substitute a Number into an Algebraic Expression

\large \begin{align} a &= 4 \\ b &= 3 \\ x &= 25 \\ y &= -8 \end{align}

## Question 1

If $a=4$ and $b=3$, evaluate $ab$.

\begin{align} ab &= a \times b \\ &= 4 \times 3 \\ &= 12 \end{align}

## Question 2

If $a=3$ and $b=5$, evaluate $a^2+b$.

\begin{align} a^2+b &= 3^2+5 \\ &= 9+5 \\ &= 14 \end{align}

## Question 3

If $a=6$ and $b=2$, evaluate $2a-5b$.

\begin{align} 2a-5b &= 2 \times 6-5 \times 2 \\ &= 12-10 \\ &= 2 \end{align}

## Question 4

If $a=-2$ and $b=-3$, evaluate $-a^2b$.

\begin{align} -a^2b &= -(-2)^2 \times (-3) \\ &= -4 \times (-3) \\ &= 12 \end{align}

## Question 5

If $a=-1$ and $b=-2$, evaluate $a^2-b^2$.

\begin{align} a^2-b^2 &= (-1)^2-(-2)^2 \\ &= 1-4 \\ &= -3 \end{align}

## Question 6

If $a=-1$ and $b=-2$, evaluate $\displaystyle \frac{a+b}{a-b}$.

\begin{align} \displaystyle \frac{a+b}{a-b} &= \frac{(-1)+(-2)}{(-1)-(-2)} \\ &= \frac{-1-2}{-1+2} \\ &= \frac{-3}{1} \\ &= -3 \end{align}