Determine la suma indicada de las siguientes progreciones 15 + 17 + 19 + 21 + ?

<img src="https://tex.z-dn.net/?f=%5Cdisplaystyle%20S%3D15%2B17%2B19%2B21%2B...%2B55%20%5C%5C%5B2pt%5D%0A%5Ctext%7BEs%20una%20serie%20aritm%5C%27etica%20de%20raz%5C%27on%20%7Dr%3D2%20%5C%5C%5B4pt%5D%0An%3D%5Cfrac%7Ba_n-a_1%7D%7Br%7D%2B1%20%5C%20%5CRightarrow%20%5C%20n%3D%5Cfrac%7B55-15%7D%7B2%7D%2B1%3D21%20%5C%20%20%2C%20%5C%20%5Ctext%7Bhay%2021%20t%5C%27erminos%7D%20%5C%5C%5B2pt%5D%0A%5Ctext%7BPara%20la%20suma%20en%5C%27esima%20se%20usa%20la%20f%5C%27ormula%20%3A%7D%20%5C%5C%5B4pt%5D%0AS_n%3D%5Cleft%28%5Cfrac%7Ba_1%2Ba_n%7D%7B2%7D%5Cright%29n%20%5C%5C%5B4pt%5D%0AS_%7B21%7D%3D%5Cleft%28%5Cfrac%7B15%2B55%7D%7B2%7D%5Cright%29%2821%29%20%5C%5C%5B4pt%5D%0AS_%7B21%7D%3D%2835%29%2821%29%20%5C%5C%5B4pt%5D%0AS_%7B21%7D%3D735%20%5C%20%5C%20%5Cleftarrow%20%5C%20Respuesta.%0A" />.