Respuesta :
The nth terms of sequence 1 ; 5 ; 11 ; 19 is n² + n + -1 and the nth terms of sequence -2;-3; -6;-11; is -n² + 2n -3 .
What is quadratic sequence?
The differences (referred to as first differences) between every two successive terms are the same, then it is called an arithmetic sequence (which is also known as a linear sequence). But if the first differences are NOT same, and instead, the second differences are the same, then the sequence is known as a quadratic sequence.
According to the question
The sequence 1 , 5 , 11 , 19
First difference : 4 6 8
Second difference: 2 2
1.1.1>
Now, expression for the nth term of the sequence.
Quadratic equation
ax² + bx + c
at n= 1
a + b + c = 1 -----------------(1)
At n = 2
4a + 2b + c = 5 ------------------(2)
At n = 3
9a + 3b + c = 11 -------------------(3)
solving equation
Subtracting equation (3) from (2)
9a + 3b + c = 11
-4a - 2b - c = -5
5a + b = 6 ----------------------(4)
Subtracting equation (2) from (1)
4a + 2b + c = 5
-a - b - c = -1
3a + b = 4 ---------------------------- (5)
Subtracting equation (5) from (4)
5a + b = 6
-3a - b = -4
2a = 2
a = 1
Putting value in equation (5)
3a + b = 4
3*1 + b = 4
b = 1
Substituting the value in equation (1)
a + b + c = 1
1 + 1 + c = 1
c = -1
Substituting the value in the quadratic equation
x² + x + -1
For nth term
n² + n + -1
1.1.2>
Term of the sequence is 2549
Now,
using the nth term
n² + n + -1 = 2549
n (n+1) = 2550
n (n+1) = 50 * 51
n = 50
1.2> sequence of numbers is given: -2 ,-3 ,-6 ,-11
First difference = -1 -3 -5
Second difference = -2 -2
1.2.1 >
Now , expression for the nth term of the sequence.
Quadratic equation
ax² + bx + c
at n= 1
a + b + c = -2 -----------------(1)
At n = 2
4a + 2b + c = -3 ------------------(2)
At n = 3
9a + 3b + c = -6 -------------------(3)
solving equations
Subtracting equation (3) and (2)
9a + 3b + c = -6
-4a - 2b - c = 3
5a +b = -3 ----------------(4)
Subtracting equation (2) and (1)
4a + 2b + c = -3
-a - b - c = 2
3a + b = -1 -------------------(5)
Now,
Subtracting equation (5) and (4)
5a +b = -3
-3a - b = 1
2a = -2
a = -1
Putting value of a in equation (5)
3*-1 + b = -1
b = 2
Substituting values in equation (1)
-1 + 2 + c = -2
c = -3
nth term
-n² + 2n -3
1.2.2>the sequence of numbers will never contain a positive term.
because its first term is -ve and difference between consecutive term is always -ve. or as per nth term n² is a -ve term .
Hence, the nth terms of sequence 1 ; 5 ; 11 ; 19 is n² + n + -1 and the nth terms of sequence -2;-3; -6;-11; is -n² + 2n -3 .
To know more about Quadratic equation here:
https://brainly.com/question/2263981
#SPJ3
