There are two sets A and B of three numbers in A.P. whose sum is 15 and where D and d are the common differences such that D−d=1. Find the numbers if Pp=78 where P and p are products of the numbers in the two sets.
A
D=2, d=1
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B
D=5, d=3
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C
D=7, d=1
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D
D=32, d=10
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Solution
The correct option is B D=2, d=1 For the A.P having common difference D we can assume its three terms (A−D)+A+(A+D)
sum of the three terms is 15 so solving the equation
(A−D)+A+(A+D)=15
∴A=5
For the A.P having common difference d we can assume its three terms (A−d)+A+(A+d)
sum of the three terms is 15 so solving the equation
(A−d)+A+(A+d)=15
∴A=5
Set A=5−D,5,5+Dand
set B=5−d,5,5+d,
D−d=1
also Pp=78
we get
5(25−D2)5(25−d2)=78
∴25(8−7)=8(d+1)2−7d2 or
25=d2+16d+8
so solving the above quadratic equation in d we get