A) 33%
B) 40%
C) 66%
D) 77%
Correct Answer: B
Solution :
We know that \[{{S}_{nth}}=u+\frac{1}{2}a(2n-1)\] \[{{S}_{3rd}}=0+\frac{1}{2}a(2\times 3-1)=\frac{5}{2}a\,\]\[(\text{for}\,n=3s)\] \[{{S}_{4th}}=0+\frac{1}{2}a(2\times 4-1)=\frac{7}{2}a\,\]\[(\text{for}\,n=4s)\] So, the percentage increase \[=\frac{{{S}_{4th}}-{{S}_{3rd}}}{{{S}_{3rd}}}\times 100\] \[=\frac{\frac{7}{2}a-\frac{5}{2}a}{\frac{5}{2}a}\times 100\] \[=\frac{\frac{2a}{2}}{\frac{5}{2}a}\times 100=2\times 20=40%\]
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