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If sum of four numbers in A.P. is 28 and product of two middle terms is 45, then product of the first and last terms is
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a
11
b
13
c
15
d
17
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Correct option is B
Let four numbers in A.P. bea−3d, a−d, a+d, a+3dWe are given 28=(a−3d)+(a−d)+(a+d)+(a+3d)=4a⇒ a=7Also, (a−d)(a+d)=45⇒ 49−d2=45⇒d=±2.Now, (a−3d)(a+3d)=a2−9d2=13Similar Questions
If and 1 are in A.P, then x is equal to
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