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Find the sum of the first 10 terms of an AP where the first term is 5 and the common difference is 2.
120
130
140
150
140
Since the sequence is 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, the sum is simply the average of the first and last term multiplied by the number of terms: 25+23тАЛ├Ч10=14├Ч10=140.
First term a = 5, common difference d = 2, number of terms n = 10
SnтАЛ=2nтАЛ[2a+(nтИТ1)d]
Since the sequence is 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, the sum is simply the average of the first and last term multiplied by the number of terms: 25+23тАЛ├Ч10=14├Ч10=140.
Students often use the formula SnтАЛ=2nтАЛ(a+l) but accidentally substitute l as a+nd instead of a+(nтИТ1)d.
Identify parameters
Identify the values provided for the arithmetic progression: the first term a=5, common difference d=2, and number of terms n=10.
a=5,d=2,n=10
Apply sum formula
Substitute the given values into the formula for the sum of n terms of an AP: SnтАЛ=2nтАЛ[2a+(nтИТ1)d].
S10тАЛ=210тАЛ[2(5)+(10тИТ1)2]
Calculate the result
Perform the arithmetic operations inside the brackets: 10+(9├Ч2)=10+18=28. Then multiply by 210тАЛ=5.
S10тАЛ=5├Ч[10+18]=5├Ч28=140
C is correct because the sum of the first 10 terms of the given arithmetic progression is calculated to be 140.
This concept is foundational for calculating series in Calculus and understanding the financial math behind simple interest sequences.