Question

P.A (12,16,20,24,28...) calcule a soma dos quinze primeiros números?
AJUDA PORFVR

Answer 1

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$\boxed{\sf{\underline{soma~dos~termos~da~PA}}}\\\huge\boxed{\boxed{\boxed{\boxed{\boxed{\sf S_n=\dfrac{n\cdot(a_1+a_n)}{2}}}}}}$

$\sf r=16-12=4\\\sf a_{15}=a_5+10r\\\sf a_{15}=28+10\cdot4\\\sf a_{15}=28+40\\\sf a_{15}=68\\\sf S_{15}=\dfrac{15\cdot(12+68)}{2}\\\sf S_{15}=\dfrac{15\cdot\diagup\!\!\!\!80}{\diagup\!\!\!2}\\\sf S_{15}=15\cdot40\\\huge\boxed{\boxed{\boxed{\boxed{\sf S_{15}=600\checkmark}}}}$