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Find the sum 5 × 7 + 9 × 11 + 13 × 15 + ….. upto n terms.

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5 × 7 + 9 × 11 + 13 × 15 + ….. upto n terms

Now, 5, 9, 13, … are in A.P. with

rth term = 5 + (r – 1) (4) = 4r + 1

7, 11, 15, ….. are in A.P. with

rth term = 7 + (r – 1) (4) = 4r + 3

∴ 5 × 7 + 9 × 11 + 13 × 15 + …… upto n terms

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