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Question
The value of 1 - 2 + 3 - 4 + 5 - 6 +........... + 19 - 20 is __________.
The value of 1 - 2 + 3 - 4 + 5 - 6 +........... + 19 - 20 is __________.
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Solution
The correct option is C -10
Here,
(1−2)=−(2−1)=−1
(3−4)=−(4−3)=−1
.
.
(19−20)=−(20−19)=−1.
There are 10 pairs having a value -1.
∴1−2+3−4+5−6 +...........+19−20
= +10×(−1)= −10 [∵x×(−y)=−xy]
-10
Here,
(1−2)=−(2−1)=−1
(3−4)=−(4−3)=−1
.
.
(19−20)=−(20−19)=−1.
There are 10 pairs having a value -1.
∴1−2+3−4+5−6 +...........+19−20
= +10×(−1)= −10 [∵x×(−y)=−xy]
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