Question 604611
how you get the sum of all the whole natural numbers greater than 89 and less than 101.
<pre>
One way is just to add up the 11 numbers:

90+91+92+93+94+95+96+97+98+99+100 = 1045

The other way is to use the sum formula for an arithmetic sequence:

S<sub>n</sub> = {{{n/2}}}(a<sub>1</sub> + a<sub>n</sub>) 

where n = 11, 

S<sub>11</sub> = {{{11/2}}}(a<sub>1</sub> + a<sub>11</sub>)

then use

a<sub>1</sub> = 90, and a<sub>11</sub> = 100

S<sub>11</sub> = {{{11/2}}}(90 + 100)

S<sub>11</sub> = {{{11/2}}}(190)

S<sub>11</sub> = {{{11/2}}}(190) = 1045.

Edwin </pre>