a=3 S4=51( S8−S4)⇒5 S4=S8−S4⇒6 S4=S8⇒6⋅24[2×3+(4−1)×d]=28[2×3+(8−1)d]⇒12(6+3 d)=4(6+7 d)⇒18+9 d=6+7 d⇒ d=−6 S20=220[2×3+(20−1)(−6)]=10[6−114]=−1080
If the first term of an A.P. is 3 and the sum of its first four terms is equal to one-fifth of the sum of the next four terms, then the sum of the first 20 terms is equal to
Held on 23 Jan 2025 · Verified 6 Jul 2026.
−1080
−1020
−1200
−120
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