Arithmetic sequences

1003057906

Level: 
B
The sum of the first \( 15 \) terms of an arithmetic sequence is \( 210 \) and the \( 15 \)th term is \( 7 \). Choose the correct formula of calculation of the first term.
\( a_1=\frac2{15}\cdot210-7 \)
\( a_1=\frac2{15}\cdot\frac{210}7 \)
\( a_1=\frac{15}2\cdot210-7 \)
\( a_1= \frac{210}7 \)
\( a_1=\frac2{15} (210-7) \)

1003057903

Level: 
A
The \( 17 \)th term of an arithmetic sequence is \( 77 \) and the common difference is \( 9 \). Choose the correct formula of calculation of the \( 5 \)th term.
\( a_5=77-12\cdot9 \)
\( a_5=17-12\cdot9 \)
\( a_5=12\cdot9-77 \)
\( a_5=77-16\cdot9 \)
\( a_5=77+12\cdot9 \)