Question 2 (i) Find the sums given below (i) 7+1012+14+……+84
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Solution
(i) For this A.P., a = 7 an=84 d=a2−a1=1012−7=212−7=72 Let 84 be the nthterm of this A.P. an=a+(n−1)d84=7+(n−1)×7277=(n−1)×7222=n−1n=23Weknowthat,Sn=n2(a+an)S23=232(7+84)=(23×912)=20932=104612