find the sum of the multiples of 3 from 3 to 99 inclusive

Multiples of 3 from 3 to 99 inclusive is a Aritmetic Progression where:a1 = 3 an = 99r = 3Using the formula of general terms:[tex]a_n=a_1+(n-1).r\\
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99=3+3(n-1)\\
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99-3=3n-3\\
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3n=99\\
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n=33[/tex]Calculating sum:[tex]S_{33}=\frac{33(a_1+a_{33})}{2}\\
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S_{33}=\frac{33(3+99)}{2}=\frac{33*102}{2}=1683[/tex]