1. Sum of the series $$1^{2} - 2^{2} + 3^{2} - 4^{2} + ... + 2001^{2} - 2002^{2} + 2003^{2}$$ is:





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  • By: anil on 05 May 2019 02.40 am
    The given series is $$1^2 - 2^2 + 3^2 - 4^2 +.....+2003^2$$
    $$1^2 - 2^2$$ can be written as $$(1+2)(1-2)$$ = $$3*(-1)$$ = $$-3$$
    $$3^2 -4^2$$ can be written as $$(3+4)*(3-4)$$ = $$7*(-1)$$ = $$-7$$
    $$5^2-6^2$$ can be written as $$(5+6)*(5-6)$$ = $$11*(-1)$$ = $$-11$$
    Therefore, all the terms till $$2002^2$$ can be expressed as an AP.
    The last term of the AP will be $$(2001+2002)(2001-2002)$$ = $$-4003$$

    Therefore, the given expression is reduced to $$-3 - 7 ...-4003 + 2003^2$$
    Let is evaluate the value of $$-3 - 7 ...-4003 $$
    Number of terms,$$n$$ = $$frac{4003-3}{4} + 1$$ = $$1001$$
    Sum = $$frac{n}{2} *$$(first term + last term)
           = $$frac{1001}{2}*(-4006)$$
           = $$ -2005003$$
    $$2003^2 = 4012009$$
    Value of the given expression = $$4012009 - 2005003 = 2007006$$.

    Therefore, option A is the right answer.
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