Answer:
-98
Step-by-step explanation:
General form of an arithmetic sequence:
[tex]a_n=a+(n-1)d[/tex]
where:
- [tex]a_n[/tex] is the nth term
- a is the first term
- d is the common difference between terms
Given:
To find the 43rd term we need to construct an equation for the nth term. To do this, find the first term and the common difference by substituting the given information into the general formula to create a system of equations that can be solved.
[tex]\begin{aligned} \underline{\textsf{Equation 1}}} \\ a_{21} & =-32 \\ \implies a+(20-1)d & = -32 \\a+20d & =-32 \end{aligned}[/tex]
[tex]\begin{aligned} \underline{\textsf{Equation 2}}} \\ a_{50} & =-119 \\ \implies a+(50-1)d & = -119 \\a+49d & =-119 \end{aligned}[/tex]
Subtract Equation 1 from Equation 2 to eliminate a:
[tex]\begin{array}{r l}a+49d & = -119\\- \quad a+20d & = -32\\\cline{1-2}29d & = -87\end{aligned}[/tex]
Solve for d:
[tex]\implies d=\dfrac{-87}{29}=-3[/tex]
Substitute the found value of d into one of the equations and solve for a:
[tex]\implies a + 20(-3)=-32[/tex]
[tex]\implies a=28[/tex]
Substitute the found values of a and d into the general formula to create an equation for the nth term:
[tex]\implies a_n=28+(n-1)(-3)[/tex]
[tex]\implies a_n=31-3n[/tex]
Finally, substitute n = 43 into the found formula to find the 43rd term of the sequence:
[tex]\implies a_{43}=31-3(43)=-98[/tex]
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