find the sum of first 20 term of the A.P. 7,11,15,19,...
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Step-by-step explanation:
AP. 7,11,15,19,...
a=7 , d= 11-7 = 4
S20= 20/2[ 2(7)+(20-1)4]
= 10[ 14+19(4)]
=10[14+76]
= 10 x 90
= 900
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