HI, I need to understand about the Arithmetic Sequence Formula. There are tons of instructions on the Internet but I still can't understand this formula. Can someone please help me explain? I want an explanation about the whole formula. eg: Why is it n-1?

A3cm×3cm\greend{3\,\text{cm} \times 3\,\text{cm}} 3cm×3cm start color greend, 3, space, c, m, times, 3, space, c, m, end color greend rectangle sits inside a circle with radius of 4cm\blued{4\,\text{cm}} 4cm start color blued, 4, space, c, m, end color blued . what is the area of the shaded region? round your final answer to the nearest hundredth.