Find the sum

Algebra Level 2

The sum of the first 5 terms of an arithmetic progression is 30.
The sum of the sixth to tenth terms of the same progression is 80.
What is the sum of the 10 0 th 100^\text{th} to 15 0 th 150^\text{th} terms?

12750 none of these 12550 12500

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1 solution

The formula for the sum of terms of an arithmetic progression is s = n 2 ( a 1 + a n ) s = \frac{n}{2}(a_1 + a_n)

Considering the first five terms, we have

30 = 5 2 ( a 1 + a 5 ) 30 = \frac{5}{2}(a_1 + a_5)

After simplifying, it becomes

a 1 + a 5 = 12 a_1+a_5 = 12

But we know that

a 5 = a 1 + 4 d a_5 = a_1 + 4d

After substituting and simplifying, we get

2 a 1 + 4 d = 12 2a_1+4d=12 ( 1 ) \color{#D61F06}(1)

Considering the first 10 10 terms, we have

s ( 1 10 ) = 30 + 80 = 110 s_{(1-10)} = 30 + 80 = 110

110 = 10 2 ( a 1 + a 10 ) 110 = \frac{10}{2}(a_1+a_{10})

After simplifying, it becomes

a 1 + a 10 = 22 a_1+a_{10}=22

But we know that

a 10 = a 1 + 9 d a_{10}=a_1+9d

After substituting and simplifying, we get

2 a 1 + 9 d = 22 2a_1+9d=22 ( 2 ) \color{#D61F06}(2)

Subtract ( 1 ) \color{#D61F06}(1) from ( 2 ) \color{#D61F06}(2) , we get

5 d = 10 5d = 10

d = 2 d = 2

Solving for a 1 a_1 by substituting 2 2 for d d in ( 1 ) \color{#D61F06}(1) or ( 2 ) \color{#D61F06}(2) , we get

2 a 1 + 4 d = 12 2a_1+4d=12 \implies 2 a 1 + 4 ( 2 ) = 12 2a_1+4(2)=12 \implies a 1 = 2 a_1 = 2

or

2 a 1 + 9 d = 22 2a_1+9d=22 \implies 2 a 1 + 9 ( 2 ) = 22 2a_1+9(2)=22 \implies a 1 = 2 a_1 = 2

Solving for a 100 a_{100} and a 150 a_{150} , we have

a 100 = a 1 + 99 d = 2 + 99 ( 2 ) = 200 a_{100} = a_1 + 99d = 2 + 99(2) = 200

a 150 = a 1 + 149 d = 2 + 149 ( 2 ) = 300 a_{150} = a_1 + 149d = 2 + 149(2) = 300

Solving for the sum of a 100 a_{100} to a 150 a_{150} , we have

s ( 100 150 ) = 51 2 ( 200 + 300 ) = s_{(100-150)} = \frac{51}{2}(200+300) = 12750 \boxed{\large\color{#20A900}12750}

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