The sum of the first 20 terms of an arithmetic progression is 405 and the sum of the first 40 terms is 1410. Find the 60th term of the progression. ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................ ............................................................................................................................................................
Exam No：9709_s21_qp_11 Year：2021 Question No：2
Answer:
\(
\begin{array}{l}
10(2 a+19 d)=405 \\
20(2 a+39 d)=1410
\end{array}
\)
Solving simultaneously two equations obtained from using the correct sum formulae \([a=6, d=1.5]\)
Using the correct formula for 60 th term with their \(a\) and \(d\)
60 th term \(=94.5\)
\begin{array}{l}
10(2 a+19 d)=405 \\
20(2 a+39 d)=1410
\end{array}
\)
Solving simultaneously two equations obtained from using the correct sum formulae \([a=6, d=1.5]\)
Using the correct formula for 60 th term with their \(a\) and \(d\)
60 th term \(=94.5\)
Knowledge points:
1.6.3 use the formulae for the nth term and for the sum of the first n terms to solve problems involving arithmetic or geometric progressions (Including knowledge that numbers a,b,c are 'in arithmetic progression' if 2 b=a+c (or equivalent) and are 'in geometric progression' if (or equivalent) (Questions may involve more than one progression.)
Solution:
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