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Please i need help with this asap

Please i need help with this asap-example-1

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\text{First term,} ~a = 3\\ \\\text{Last term,}~ l = 69\\\\\text{Common difference,} ~d= 9-3 =6\\\\\\\text{Number of terms, n =?}\\\\~~~~~~~l=a+(n-1)d\\\\\implies 69=3+(n-1)6 \\\\\implies 69 = 6n-3\\\\\implies 6n = 72\\\\\implies n = \frac{72}6 =12\\\\\text{Sum of finite arithmetic series,}\\\\S_n= \frac n2 (a+l) = \frac{12} 2 (3+69) = 6* 72 = 432

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