Question

find the sum of the first 20 terms of the A.P 7,11 ,15,19,...


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Answer 1

given

• Ap - 7 ,11,15,19.

To find

• sum of first 20 terms

solution

first term of ap is -7

common difference of Ap is

• a2-a1=11-7=4
• a3-a2=15-11=4

now formula of finding sum is

s(n)=n/2(2a+(n-1)*d

put value

=20/2(2(7)+(20-1)*4)

=10(14+76)

=10(90)

=900

hence,the sum of first 20 term is

$\huge \underline \frak \pink {900}$

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Answer 2

given

Ap - 7 ,11,15,19.

To find

sum of first 20 terms

solution

first term of ap is -7

common difference of Ap is

a2-a1=11-7=4

a3-a2=15-11=4

now formula of finding sum is

s(n)=n/2(2a+(n-1)*d

put value

=20/2(2(7)+(20-1)*4)

=10(14+76)

=10(90)

=900

hence,the sum of first 20 term is

\huge \underline \frak \pink {900}

900