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S1. S2, S respectively. Prove that:26. The sum of n, 2n, 3n terms of an A.P:, are27 A train travels a distance of 480 km at a uniform speed. If the speed had been 8 km/h less, thenS3 3(82-S1). |
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Answer» Sol:Let ‘a’ be the first term of the AP and ‘d’ be the common differenceS1= (n/2)[2a + (n – 1)d] --- (1)S2= (2n/2)[2a + (2n – 1)d] = n[2a + (n – 1)d] --- (2)S3= (3n/2)[2a + (3n – 1)d] --- (3)Consider the RHS: 3(S2– S1)= S3= L.H.S∴ S3= 3(S2- S1) . |
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