Answer:
[tex]a = -9[/tex]
[tex]b = 4[/tex]
Step-by-step explanation:
Given
Sequence: a+3b, a+7b, a+11b
2nd term = 19
5th term = 67
Required
Find a and b
First, the 5th term needs to be calculated;
Using formula for Arithmetic Progression (AP), the formula goes thus
[tex]T_n = T_1 + (n - 1)d[/tex]
Where n = 5
T_1 = a + 3b ------------ FIrst term
[tex]d = T_2 - T_1 or T_3 - T_2[/tex] --- Difference between two successive terms
[tex]d = a + 7b - (a + 3b)[/tex]
[tex]d = a + 7b -a - 3b[/tex]
[tex]d = a - a + 7b - 3b[/tex]
[tex]d = 4b[/tex]
So, [tex]T_n = T_1 + (n - 1)d[/tex] becomes
[tex]T_5 = a + 3b + (5 - 1)4b[/tex]
[tex]T_5 = a + 3b + (4)4b[/tex]
[tex]T_5 = a + 3b + 16b[/tex]
[tex]T_5 = a + 19b[/tex]
Now that we have values for 2nd and 5th term;
From the second, T2 = 19 and T5 = 67
This gives
[tex]a + 7b = 19[/tex] --- Equation 1
[tex]a + 19b = 67[/tex] ---- Equation 2
Make a the subject of formula in (1)
[tex]a = 19 - 7b[/tex]
Substitute these values in equation 1
[tex]a + 19b = 67[/tex] becomes
[tex]19 - 7b + 19b = 67[/tex]
[tex]19 + 12b = 67[/tex]
Collect like terms
[tex]12b= 67 - 19[/tex]
[tex]12b = 48[/tex]
Divide both sides by 12
[tex]\frac{12b}{12} = \frac{48}{12}[/tex]
[tex]b = 4[/tex]
Recall that b = 4
Substitute a = 19 - 7b and nothing will hire
[tex]a = 19 - 7(4)[/tex]
[tex]a = 19 - 28[/tex]
[tex]a = -9[/tex]
Hence, the values of a and b are -9 and 4 respectively.
