Question

The nth term of an A.P.is given by (-4n + 15). Find the sum of first 20 terms of this A.P.​

Answer 1

Explanation:

$An = -4n + 15$

$A1 = a = (-4 + 15) = 11$

$A(20)=-4(20)+15$

$A(20)=-80+15 = -65$

$Sum \: of \: first \: 20 \: terms-$

$\small\boxed{S(n) = \frac{n}{2}[a + An]}$

$S(20) = \frac{20}{2}[a + A(20)]$

$S(20) = 10[11 + (-65)]$

$S(20) = 10*(-54)$

$S(20) = -540$

Answer 2

Answer:

Explanation:

An = -4n + 15An=−4n+15

A1 = a = (-4 + 15) = 11A1=a=(−4+15)=11

A(20)=-4(20)+15A(20)=−4(20)+15

A(20)=-80+15 = -65A(20)=−80+15=−65

$Sum \: of \: first \: 20 \: terms-Sumoffirst20terms−</p><p></p><p>\small\boxed{S(n) = \frac{n}{2}[a + An]} </p><p>S(n)= </p><p>2</p><p>n</p><p> </p><p> [a+An]</p><p> </p><p> </p><p></p><p>S(20) = \frac{20}{2}[a + A(20)]S(20)= </p><p>2</p><p>20</p><p> </p><p> [a+A(20)]$

S(20) = 10[11 + (-65)]S(20)=10[11+(−65)]

S(20) = 10*(-54)S(20)=10∗(−54)

S(20) = -540S(20)=−540