Can someone please explain this question..Please log in to comment and participate in discussions.
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IPMAT/Algebra
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Each term ai·ai+1 is either +1 or −1, so the sum is just n such terms. For the total to be 0, the number of +1 and −1 terms must be equal, so n must be even. Now if you multiply all terms, everything cancels nicely and you get a1²·a2²·…·an² = 1, which means the number of −1 terms must be even. Since the number of −1 terms is n/2, that must be even, so n is a multiple of 4.
[p/synicdx](/forums/p/synicdx) bhaiya would you recommend watching JEE lectures (specifically chincholikar sir's) for progression and series? or just CAT lectures are sufficient to cover the concepts?
But how do we know that on multiplying the terms it will be 1 only and not -1?