Question

if an ap if a=1 an=20 and sn=42 <br />then n is​

Answer 1

please refer to the attachment!!

Answer 2

Given :-

• a = 1
• ${a}_{n}$ = 20
• ${S}_{n}$ = 42

❤${S}_{n}$ = $\frac{a \: + \: {a}_{n} \: }{ 2 }$ × n

• 42 = $\frac{ 1 \: + \: 20 }{ 2 }$ × n

• 42 = $\frac{ 21 }{ 2 }$ × n

• 21 n = 42 × 2

• n = $\frac{ 42 \: × \: 2 }{ 21 }$

• n = ${ 2 \: × \: 2 }$

❤$\huge\fbox\green{ n \: = \: 4 }$