Question

What is the value of 1/15 + 1/35 + 1/63 + 1/99 + 1/143 ?

A 6/23 Correct Answer Incorrect Answer
B 5/39 Correct Answer Incorrect Answer
C 4/19 Correct Answer Incorrect Answer
D 8/41 Correct Answer Incorrect Answer

Solution

1/(3×5) + 1/(5×7) + 1/(7×9) + 1/(9×11) + 1/(11×13) ⇒ (total terms)/(smallest term × largest term) ⇒ 5/(3×13) = 5/39 Alternate method: 1/(3×5) + 1/(5×7) + 1/(7×9) + 1/(9×11) + 1/(11×13) = 1/(difference ) {T1-Tlast }= 1/2 {1/3-1/13}=1/2 {10/39}=5/39

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