Question

Please help to prove?

Answer 1

See below

Explanation:

Lets deploy the left side:

`sum_(j=1)^6 3(j+4)=3(1+4)+3(2+4)+3(3+4)+..+3(6+4)`.

We have a factor of 3 in all sumands. We can extract common factor 3

`sum_(j=1)^6 3(j+4)=3(1+4)+3(2+4)+3(3+4)+..+3(6+4)=3[(1+4)+(2+4)+...+(6+4)]`. This is the same to `3·sum_(j=1)^6 (j+4)`