Arithmetic and geometric sequences formulas

Must learn this equation as it is not given on the formula booklet

u_n = a + (n - 1) d

where

• u_n is the n^th term
• a is the first term
• d is the common difference

Given in formula booklet (though you need to learn what the letters mean)

S_n = 1/2 n (2a + (n − 1) d)

where

• n is the number of terms you are adding
• a is the first term
• d is the common difference

Must learn this equation as it is not given on the formula booklet

u_n = a r ^{(n - 1)}

where

• u_n is the n^th term
• a is the first term 
• r is the common ratio

Given in formula booklet (though you need to learn what the letters mean)

S_n = a (1 - r ⁿ ) / 1 - r

where 

• a is the first term
• r is the common ratio
• n is the number of terms added

Sum to infinity = a / 1 - r

where

• a is the first term
• r is the common ratio if it is less than 1