Question

a=7 , l=49 , Sn =420 find d​

Answer 1

Answer:

first term(f)=7

last term(l)=49

common difference=d

n(number of terms)=n

sn(sum)=420

sn=

\begin{gathered} \frac{n}{2} (f+l) \\ \\ 420= \frac{n}{2}(7+49) \\ \\ 840=n*56 \\ 56n=840 \\ n= \frac{840}{56}=15\end{gathered}

2

n

(f+l)

420=

2

n

(7+49)

840=n∗56

56n=840

n=

56

840

=15

number of terms=15

n=

\begin{gathered} \frac{l-f}{d}+1 \\ \\ 15= \frac{49-7}{d}+1 \\ \\ 15= \frac{42}{d} +1 \\ \\ 14= \frac{42}{d} \\ \\ 14d=42 \\ d= \frac{42}{14}=3 \end{gathered}

d

l−f

+1

15=

d

49−7

+1

15=

d

42

+1

14=

d

42

14d=42

d=

14

42

=3

common difference=3

Answer 2

Answer:

Step-by-step explanation:

sn=n/2(a+l)

420=n/2(7+49)

420=n/2(56)

420=n(28)

n=420/28

n=15