Question

Find the sum of first 20 terms of an Arithmetic Progression
whose first term is 3 and common difference is 2.​

Answer 1

$\large\bf\underline{Given:-}$

• First term of AP = 3
• common difference = 2

$\large\bf\underline {To \: find:-}$

• sum of 20 terms.

$\huge\bf\underline{Solution:-}$

• first term = 3
• common difference = 2

$\large \bf \: s_n = \frac{ n}{2} [2a + (n - 1)d]$

$\rm \: s_{20} = \frac{20}{2} (2 \times 3 + (20 - 1)2) \\ \\ \rm \: s_{20} = 10(6 + 19 \times 2) \\ \\ \rm \: s_{20} = 10( 6+ 38) \\ \\ \rm \: s_{20} = 10(44) \\ \\ \bf \: s_{20} = 440$

so, sum of 20 terms of AP is 440.

━━━━━━━━━━━━━━━━━━

Some important formulas : -

• an = a+(n-1)d
• sn = n/2(a+l)
• sn = n/2 ×[2a + (n -1 ) d]

where,

• sn = sum of terms
• a = First term
• d = common difference
• n = number of terms.
• l = last term
• an = nth term

━━━━━━━━━━━━━━