this is a example from my lecture notes and while studying i cannot understand how the final answer is found. the particular place where i am confused i have circled. shouldn't it be -1/n(n+1) ?

this is a example from my lecture notes and while studying i cannot understand how the final answer is found. the particular place where i am confused i have circled. shouldn't it be -1/n(n+1) ?

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It should be $ \displaystyle\frac1{n(n+1)}. $