Answer:
4670
Step-by-step explanation:
that is the workings above
If the third term is 8 and eighth term is 143 then the sum of first 20 terms is 4210.
Arithmetic progression is a sequence in which all the terms have equal differences. Nth term will be a+(n-1)d in which a is first term, d is difference and n is number of term.
We have been given that third term is 8 and eight term is 143 and we have to calculate the sum of first 20 terms.
[tex]A_{3}[/tex]=a+(3-1)d
8=a+2d-----1
[tex]A_{8}[/tex]=a+(8-1)d
143=a+7d---2
Subtract equation 1 from equation 2.
a+7d-a-2d=143-8
5d=135
d=27
Put the value of d in equation 1.
a+2d=8
a+2*27=8
a+54=8
a=-46
We know that,
[tex]S_{n}[/tex]=n/2[2a+(n-1)d]
=20/2[2*(-46)+(20-1)*27]
=10(-92+19*27)
=10(-92+513)
=10*421
=4210
Hence if the third term is 8 and eighth term is 143 then the sum of first 20 terms is 4210.
Learn more about arithmetic progression at https://brainly.com/question/6561461
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