C-13-Q120mediumsingle_mcqThe sum of the first 777 terms of 2−4+8−16+⋯2 - 4 + 8 - 16 + \cdots2−4+8−16+⋯ is —a868686b−86-86−86c128128128d−128-128−128ব্যাখ্যাWith a=2a=2a=2, r=−2r=-2r=−2, n=7n=7n=7: S7=2((−2)7−1)−2−1=2(−128−1)−3=−258−3=86S_7=\dfrac{2\left((-2)^{7}-1\right)}{-2-1}=\dfrac{2(-128-1)}{-3}=\dfrac{-258}{-3}=86S7=−2−12((−2)7−1)=−32(−128−1)=−3−258=86.