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17. The number of terms of the series 54 51 + 48 +..... suchthat thesum is 513 is(i) 18(ii) 19(iii) both (i) and (ii) (iv) 1 |
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Answer» a=54d=51-54=-3 Sum=513 S=(n/2)[2a+(n-1)d]513=(n/2)[2*54+(n-1)(-3)]513*2=n[108-3n+3]1026=n[111-3n]1026=111n-3n²3n²-111n+1026=0n²-37n+342=0n²-18n-19n+342=0n(n-18)-19(n-18)=0(n-18)(n-19)=0 n=18,19 Option c is correct |
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