Given the \( n^{\text {ti }} \) term of a number sequence is \( T_{n}=n(5 n-7) \), where \( n=1,2,3,4 \),... Find the \( 9^{\text {th }} \) term of the number sequence. A 350 B 342 C 304 D 279

To find the \( 9^{\text{th}} \) term of the sequence given by \( T_{n}=n(5n-7) \), we start by substituting \( n = 9 \): \[ T_{9} = 9(5 \cdot 9 - 7) \] Calculating this step-by-step: 1. First calculate \( 5 \cdot 9 = 45 \). 2. Then subtract \( 7 \): \( 45 - 7 = 38 \). 3. Now multiply by \( 9 \): \( 9 \cdot 38 = 342 \). Thus, the \( 9^{\text{th}} \) term of the number sequence is **342**, which corresponds to option B.