Answers

2016-01-21T21:20:10+05:30
The number is 1,7,13 this is arithmetic series a=1,d=6
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2016-01-21T21:22:48+05:30
Let  a_{1} = a - d ,  a_{2} = a and  a_{3}   = a + d.
From question,
 a_{1}  a_{2}  a_{3} = 21
a - d + a + a + d = 21
3a = 21
a = 7
Now, 
 a_{1}  a_{3}   =  a_{2} + 6
(a - d)(a + d) = a + 6
(7 - d) (7 + d) = 7 + 6
 7^{2}  d^{2}  = 13
49 -  d^{2} = 13
 d^{2} = 13 - 49
 d^{2} = - 36
 d^{2} = 36
d =  \sqrt{36}  
d = 6.
Now,
 a_{1} = a - d = 7 - 6 = 1
 a_{2}  = a = 7
 a_{3} =  a + d. = 7 + 6 = 13
∴   a_{1} = 1
     a_{1} = 7
     a_{1} = 13
Please comment if answer is satisfying/
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