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Explain why 7*11*13+13a n d7*6*5*4*3*2*1...

Explain why `7*11*13+13a n d7*6*5*4*3*2*1 are composite numbers.

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`7 × 11 × 13 + 13 = 13 (7 × 11 + 1)`
`= 13(77 + 1)`
`= 13 × 78
`= 13 × 13 × 6 × 1`
...
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