1
Question
Find the value of 1 - 2 + 3 - 4 + 5 - 6 +........... + 19 - 20.
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Solution
Here,
(1−2)=−(2−1)=−1
(3−4)=−(4−3)=−1
.
.
(19−20)=−(20−19)=−1.
[1 mark]
There are 10 pairs having a value -1.
∴1−2+3−4+5−6 +...........+19−20
= +10×(−1)= −10 [∵x×(−y)=−xy]
[1 mark]
Here,
(1−2)=−(2−1)=−1
(3−4)=−(4−3)=−1
.
.
(19−20)=−(20−19)=−1.
[1 mark]
There are 10 pairs having a value -1.
∴1−2+3−4+5−6 +...........+19−20
= +10×(−1)= −10 [∵x×(−y)=−xy]
[1 mark]
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