find the sum of first 13terms if 7 term is 40
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Step-by-step explanation:
Sum of n terms =n/2(1st term+ nth term). Let the first term of an A.P. be a and common difference is d. So the 7th term of the A.P. is40=> a+6d =40. Sum of first 13 terms = 13/2(a +(a+12d))= 13/2(2a+12d) =13/2(2)(a+6d) =13(a+6d) =13(40)=520 .
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