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[tex]\displaystyle\bf\\a_{n} =5n+2\\\\a_{1} =5\cdot 1+2=5+2=7\\\\a_{15} =5\cdot 15+2=75+2=77\\\\\\S_{15} =\frac{a_{1} + a_{15} }{2} \cdot 15=\frac{7+77}{2} \cdot 15=\frac{84}{2} \cdot 15=42\cdot 15=630\\\\\\Otvet \ : \ S_{15} =630[/tex]