Question

25. Find :
1-2 +3-4+5-6+ ......... +17-18 + 19-20​

Answer 1

Answer:

Hi ,

1 -2 + 3 - 4 + .....+ 19 - 20

= ( 1+3+5+.….+19)-( 2+4+6+.…+20 )

= 10² - [ 10/2 ( 2 + 20 ) ]

= 100 - ( 10 × 11 )

= 100 - 110

= -10

Or

( 1-2) + (3 - 4 )+ ( 5 - 6 ) +...+ ( 19 - 20 )

= -1 -1 -1 -....10 times

= 10 × ( -1 )

= -10

I hope this helps you.

Explanation:

Answer 2

Explanation:

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