In an AP of 21 terms the sum of the first 3 terms is ndash 33 and that of the middle 3 is 75 What is...........

Quantitative Aptitude ( CCS) Arithmetic progression

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In an AP of 21 terms, the sum of the first 3 terms is – 33 and that of the middle 3 is 75. What is the sum of the AP?

-955

-1155

525

715

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Explanation:

The AP can be expressed as a, (a + d), ---, (a +20d).
The sum of the first 3 terms is (3a + 3d) = -33
and the sum of the middle 3 terms is (3a + 30d) = 75.
Solving these two equations, we get a = -15 and d = 4.
The sum of the 1st 21 terms is (21/2) (2 * -15 + 20 * 4) = 525.