ILL GIVE BRAINLIEST! PLEASE HELP I DONT UNDERSTAND! Use the arithmetic series formula to calculate the sum of the first 15 terms of the sequence: a n =5n+3

S₁₅ = 645

Step-by-step explanation:

The sum to n terms of an arithmetic sequence is

= [ 2a₁ + (n - 1)d ]

where a₁ is the first term and d the common difference

Using the nth term formula = 5n + 3 , then

a₁ = 5(1) + 3 = 5 + 3 = 8

a₂ = 5(2) + 3 = 10 + 3 = 13 , then

d = a₂ - a₁ = 13 - 8 = 5

Thus

S₁₅ = [ (2 × 8) + (14 × 5) ]

     = 7.5( 16 + 70)

     = 7.5 × 86

     = 645

angle addition postulate

2x=56

step-by-step explanation: