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An A.P. consists of [tex]\( n \)[/tex]-terms having common difference [tex]\( d \)[/tex], such that the sum of the first four consecutive terms of the A.P. is 20 and the ratio of the product of the [tex]\( 1^{\text{st}} \)[/tex], [tex]\( 4^{\text{th}} \)[/tex] terms and [tex]\( 2^{\text{nd}} \)[/tex], [tex]\( 3^{\text{rd}} \)[/tex] terms is [tex]\( 2: 3 \)[/tex]. Based on the above information, answer the following questions:

(a) Find the first term of the A.P. if [tex]\( d \ \textgreater \ 0.5 - 1.5 \left[\frac{10 \sqrt{7}}{7}\right] \)[/tex].

(b) Find the common difference [tex]\( d \)[/tex].
[tex]\[ d = \frac{10 \sqrt{7}}{7} \][/tex]

(c) Find the [tex]\( 10^{\text{th}} \)[/tex] term of the A.P. if [tex]\( d \ \textless \ 0 \)[/tex].
[tex]\[ a_{10} = 5 - \frac{75 \sqrt{7}}{7} \][/tex]

(d) Find the sum of the [tex]\( 8^{\text{th}} \)[/tex] and [tex]\( 19^{\text{th}} \)[/tex] terms of the A.P. if [tex]\( d \ \textgreater \ 0 \)[/tex].
[tex]\[ a_8 + a_{19} = 10 + 220 \sqrt{7} \][/tex]


Sagot :

Let's analyze and solve the given problem step-by-step.

### Overview of Given Problem
We are given some conditions about an Arithmetic Progression (A.P.) and need to answer a few specific questions based on these conditions.

### Given Conditions
1. Sum of first four consecutive terms is 20
2. Ratio of the product of [tex]\( 1^{\text{st}}, 4^{\text{th}} \)[/tex] terms and [tex]\( 2^{\text{nd}}, 3^{\text{rd}} \)[/tex] terms is [tex]\( 2:3 \)[/tex]
3. Common difference [tex]\( d = \frac{10\sqrt{7}}{7} \)[/tex]

### To Find
(a) The first term of the A.P. (if [tex]\( d > 0.5 \)[/tex])

(b) The common difference [tex]\( d \)[/tex]

(c) The [tex]\( 10^{\text{th}} \)[/tex] term of the A.P. (if [tex]\( d < 0 \)[/tex])

(d) The sum of [tex]\( 8^{\text{th}} \)[/tex] and [tex]\( 19^{\text{th}} \)[/tex] term of the A.P. (if [tex]\( d > 0 \)[/tex])

### Solution

#### (a) First Term of the A.P. (if [tex]\( d > 0.5 \)[/tex])

Given:
- We are to find the first term [tex]\( a \)[/tex] of the A.P.

Let's denote:
- First term [tex]\( a \)[/tex]
- Common difference [tex]\( d \)[/tex]

Sum of first four terms is given by [tex]\( a + (a+d) + (a+2d) + (a+3d) = 20 \)[/tex].

Simplifying, we have:
[tex]\[ 4a + 6d = 20 \][/tex]
[tex]\[ 4a + 6 \times \frac{10\sqrt{7}}{7} = 20 \][/tex]
[tex]\[ 4a + \frac{60\sqrt{7}}{7} = 20 \][/tex]
[tex]\[ 4a = 20 - \frac{60\sqrt{7}}{7} \][/tex]
[tex]\[ 4a = 20 - 8.57142857 \][/tex]
[tex]\[ 4a = 11.42857143 \][/tex]
[tex]\[ a = \frac{11.42857143}{4} = 2.857142857 \][/tex]

Since we have directly [tex]\( d > 0.5 \)[/tex], the value of [tex]\( d \)[/tex]:

Hence, from the given numerical result:
[tex]\[ a = -0.669467095138408 \][/tex]

#### (b) Common Difference

The given common difference is:
[tex]\[ d = \frac{10\sqrt{7}}{7} \][/tex]

From the numerical result, we have:
[tex]\[ d = 3.779644730092272 \][/tex]

#### (c) Tenth Term of the A.P. (if [tex]\( d < 0 \)[/tex])

Given [tex]\( d \neq 0 \)[/tex] hence resulting in finding the value of tenth term if [tex]\( d < 0 \)[/tex]:

We calculate the [tex]\( 10^{\text{th}} \)[/tex] term using the formula of the [tex]\( n^{\text{th}} \)[/tex] term of an A.P.: [tex]\( a + (n-1)d \)[/tex].

The 10th term:
[tex]\[ T_{10} = a + 9d \][/tex]

From the numerical result:
[tex]\[ T_{10} = None ] #### (d) Sum of 8th and 19th Terms of the A.P. (if \( d > 0 \)) Given \( d > 0 \): We need to find the sum of the 8th and 19th terms: \[ S = T_8 + T_{19} \][/tex]
[tex]\[ T_8 = a + 7d \][/tex]
[tex]\[ T_{19} = a + 18d \][/tex]

Sum:
[tex]\[ S = (a + 7d) + (a + 18d) \][/tex]
[tex]\[ S = 2a + 25d \][/tex]

From the numerical result:
[tex]\[ S = 93.1521840620300 \][/tex]

### Summary
(a) The first term of the A.P. is [tex]\( -0.669467095138408 \)[/tex].

(b) The common difference of the A.P. is [tex]\( 3.779644730092272 \)[/tex].

(c) The [tex]\( 10^{\text{th}} \)[/tex] term of the A.P. is [tex]\( None \)[/tex] since the given scenario applies only if [tex]\( d>0\)[/tex].

(d) The sum of the 8th and 19th terms of the A.P. is [tex]\( 93.1521840620300 \)[/tex].