Arithematic Progression

$n^{\text{th}}$ term of Arithmetic Progression

Page 62

$$t_n=a+(n-1)\cdot d$$

where,
$a$ - first term
$d$ - common difference

Sum of first $n$ terms of Arithmetic Progression

Page 69

$$S_n=\frac{n}{2}[2a+(n-1)d]$$

where,
$a$ - First term
$d$ - common difference

Page 69

$$S_n=n\cdot a+\frac{n(n-1)}{2}\cdot d$$

where,
$a$ - First term
$d$ - common difference

Page 70

$$S_n=\frac{n}{2}(t_1+t_n)$$

where,
$t_1$ - first term
$t_n$ - $n^{\text{th}}$ term

Sum of first $n$ Natural Numbers

Page 70

$$S_n=\frac{n(n+1)}{2}$$

where,
$n$ - number of terms