Trainee triangular wedge

Trainees stand on the marks in rows exactly 1.5 m apart. They form an expanding triangular wedge (in each subsequent row, there is one more exerciser), while the distance between the front exerciser and the back row is 30 m. Determine the number of trainees.

Final Answer:

s =  231

Step-by-step explanation:

$$\begin{gathered} a=30\text{m} \\ b=1.5\text{m} \\ n=a/b+1=30/1.5+1=21 \\ {a}_1=1 \\ d=1 \\ {a}_{20}=a_1+(n-1)\cdot d=1+(21-1)\cdot 1=21 \\ s=\frac{a_1+a_{20}}{2}\cdot n=\frac{1+21}{2}\cdot 21=231 \end{gathered}$$

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