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+1

868

1

+32

Let $a_1, a_2, a_3,\dots$ be an increasing arithmetic sequence of integers. If $a_4a_5 = 13$ , what is $a_3a_6$ ?

EshEs2n Jan 17, 2019

CPhill · Answer 1 · 2019-01-17T05:50:12

The only way that a4 * a5 = 13 is if a4 = 1 and a5 = 13

So....d = 12

So a3 = (a4 - d) and a6 = ( a5 + d)

So

a3 * a6 =

(a4 - d ) ( a5 + d) = ( 1 - 12) ( 13 + 12) = (-11) (25) = -275