Someone please help me with this!! <3

Um, your sum should be 4.

Step-by-step explanation: The sum of an infinite geometric series can be found using the formula where a is the first term and r is the ratio between the successive terms.

For , find the first term in the series by substituting in the lower bound and simplifying: a=1.

For r, find the ratio of successive terms by plugging into the formula and simplifying: .

For 1–r, substitute the values of the ratio and first term into the sum formula: ; ∴, your sum should be 4.

  see the attachment

Step-by-step explanation:

It helps to follow along the proof and consider how you get from here to there.

In the 5th line, we have a sum in symbols.

In the 7th line, we have an expression with a number on the right side, and a statement that says we got there by subtraction.

Then we expect line 6 to have a sum with numbers whose difference is 40°. (That eliminates the 3rd and 4th choices.)

The way you get from an expression with symbols to an expression with numbers is to make a substitution of numbers for symbols. Hence the appropriate choice is the one shown below.