SOLUTION: The sum of the n terms of two arithmetic progression are in the ratio (3n+8):(7n+15). Find the ratio of their 12th terms

Algebra.Com
Question 1150618: The sum of the n terms of two arithmetic progression are in the ratio (3n+8):(7n+15). Find the ratio of their 12th terms
Answer by ikleyn(52858)   (Show Source): You can put this solution on YOUR website!
.
The sum of the n terms of two arithmetic progression are in the ratio (3n+8):(7n+15). Find the ratio of their 12th terms
~~~~~~~~~~~~~~~~~~~

Our APs (arithmetic progressions) are

    a,  a+d,  a+2*d,  a+3*d, . . . 

    b,  b+e,  b+2*e,  b+3*e, . . . 

where  "a"  and  "b"  are first terms  and  "d"  and  "e"  are their common differences.   


In order for to answer the question, it is enough to know three ratios  ,    and  .

So, our goal now is to find these three ratios.



1)  At n= 1  we have


         =  =  = .         (1)  



2)  At n= 2  we have


         =  = ,  or

        29*(a+d) = 14*(b+e),

        29a + 29d - 14e = 14b.


    Divide both sides by "b" 

        29*(a/b) + 29*(d/b) - 14*(e/b) = 14.


    Substitute here  =  from (1) and multiply both sides by 2. You will get then

        58*(d/b) - 28*(e/b) = -1.       (2)



3)  At n= 3  we have


         =  = ,  or

        36*(a+2d) = 17*(b+2e),

        36a + 72d - 34e = 17b.


    Divide both sides by "b" 

        36*(a/b) + 72*(d/b) - 34*(e/b) = 17.


    Substitute here  =  from (1). You will get then

        72*(d/b) - 34*(e/b) = -1.       (3)



4)  Introduce new variables  D =   and  E = .  For these unknowns, from (2) and (3) you have this system of equations

        58*D - 28*E = -1        (2')

        72*D - 34*E = -1        (3')

    Solve it by any method you want / (you know).  The solution is  D = ,  E = .  Thus   = ,   = .



5)  Now we are in position to calculate   =  =  =  =  = .    ANSWER

Solved.


RELATED QUESTIONS

If the sum of n terms of two arithmetic progressions is in the ratio (3n+8): (7n+15) find (answered by solver91311,greenestamps,ikleyn)
The sum of nth of two arithmetic progression are in the ratio of (3n+8):(7n+15).find the... (answered by ikleyn)
the sum of the first n terms of two A.Ps are as 13-7n:3n+1.find the ratio of their first... (answered by ewatrrr)
The ratio between the sum of 'n' terms of two arithmetical progressions is... (answered by t0hierry)
A geometric progression has 6 terms. The first term is 192 and the common ratio is 1.5.... (answered by greenestamps)
A geometric progression has six terms. The first term is 486 and the common ratio is ;.... (answered by greenestamps)
The first term of an arithmetic progression is 12 and the sum of the first 16 terms is... (answered by greenestamps)
A geometric progression and an arithmetic progression have the same first term. The... (answered by htmentor)
(a) The number n - 4, n + 2, 3n + 1 are in geometrical progression. Find the two possible (answered by josgarithmetic,ikleyn)