Question

Find S20 for the series -1 + -3 + -5 + -7 +…

Answer 1

here is a curious pattern that might help
-1-3=-4=-\(2^2\)
-1-3-5=-9=-\(3^2\)
-1-3-5-7=-16=

can you spot the pattern?

Answer 2

@baru so i'm subtracting and then i'm multiplying by 2 ?

Answer 3

'S20' means sum of first 20 terms right?

Answer 4

yes @baru

Answer 5

this is the series
\[-1,-3,-5,-7,-9......\]

sum of first two terms
\[-1-3=-4\\=\\-2^2\]

sum of first three terms
\[-1-3-5=\\-9=\\-3^2\]

sum of first four terms\[-1-3-5-9\\=-16\\=-4^2\]

Answer 6

can you spot the pattern?

Answer 7

@baru it's going up by 1 ?

Answer 8

can you add this:
-1-3 = ?

Answer 9

@baru it's -4

Answer 10

what is -\(2^2\)

Answer 11

@baru 4

Answer 12

\[-(2^2)=-4\]

Answer 13

so can we say
-1-3
=-4
=-\(2^2\)

?

Answer 14

@baru yes