\\\texttt{Given\; that\;} \\H_k=\frac{k(k-1)}{2}cos\left ( \frac{k(k-1)\pi}{2} \right )\\\texttt{Find \;teh\; value}\; of\\\;H_{19}+H_{20}+H_{21}......H_{98}
try it..for practice
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1 Answers
62this question is veyr simple...
just observe that we have cos of integer multiples of pi...
They take values of +1, -1 depending on whether or not the number is a multipel of 2pi
SO this sum is simply.. alternating +1 of k(k-1)/2
