Question

The sum of the first three terms in a GP is 38.Their product is 1728.Find the values of the three terms.

Answer 1

Answer:

The value of the three terms is 8 and 18

Step-by-step explanation:

Let "a" be the first term and "r" be the common ratio.

Then from the condition, we have these two equations

   a + ar + ar^2  =   38,      (1)

   a*(ar*)*(ar^2) = 1728.      (2)

From equation (2),  a^3*r^3 = 1728,  or  (ar)^3 = 1728,   which implies

   ar = root%283%2C1728%29 = 12;          (3)    

hence,  

   r  = 12%2Fa.                   (4)

Now, in equation (1) replace the term  ar  by 12, based on (3).  You will get

   a + 12 + ar^2 =  38,   which implies

   a + ar^2 = 26.              (5)

Next, substitute  r = 12%2Fa  into equation (5), replacing "r" there.  You will get

   a + a%2A%28144%2Fa%5E2%29 = 26,   or

   a + 144%2Fa = 26.

Multiply by "a" both sides and simplify

   a^2 - 26a + 144 = 0,

   %28a-13%29%5E2 - 169 + 144 = 0

   %28a-13%29%5E2 = 25

   a - 13 = +/- sqrt%2825%29 = +/- 5.

Thus two solutions for "a" are  a = 13 + 5 = 18  or  a = 13 - 5 = 8.

If  a =  8, then from (4)  r = 12%2F8 = 3%2F2.

If  a = 18, then from (4)  r = 12%2F18 = 2%2F3.

   

In the first case, if a = 8,  then the three terms are  8, 8%2A%283%2F2%29 = 12  and  8%2A%283%2F2%29%5E2 = 18.

   In this case, the sum of terms is  8 + 12 + 18 = 38, so this solution does work.

In the second case, if a = 18,  then the three terms are  18, 18%2A%282%2F3%29 = 12  and  18%2A%282%2F3%29%5E2 = 8.

   In this case, the sum of terms is  18 + 12 + 8 = 38, so this solution does work, too.

ANSWER.  The problem has two solution:  

        a)  first term is 18;  the common difference is 2%2F3  and the progression is  18, 12, 8.

        b)  first term is  8;  the common difference is 3%2F2  and the progression is   8, 12, 18.