Question

Find the value of 1-2+3-4+5-6+7-8+9 .. . . . . . .. . .+29-30
can any one help me my exam is in a few hrs.

Answer 1

Answer:

-15

Step-by-step explanation:

1-2+3-4+....+29-30

here (1+3+5+....+29) - (2+4+8+...+30)

      $S_{n} = \frac{n}{2}(a +L)$  

 L = last term of series and a is first term

     using arithmetic progression  to find sum for both n= 15

   = $(\frac{15}{2} (1+29) ) - ((\frac{15}{2}(2+30 ))$

    = $\frac{15}{2}$× 30  -

   = 15 ×15 - 15 × 16

    =15 (15-16)

    = 15× (-1)

    = -15

I hope it will help you!