Answer:
Step-by-step explanation:
t1 = a1
t2 = a1 + d
t3 = a1 + 2d
The sum is t1 + t2 + t3 = a1 + a1 + d + a1 + 2d = 3a1 + 3d
Second question
t1 = a1
t2 = a1 + d
t3 = a1 + 2d
t4 = a1 + 3d
...
tn = a1 + (n - 1)*d
Sum = (a1 + a1 + (n- 1)*d) * n / 2
Sum = (2a1 + nd - d) * n / 2
Sum = (2a1*n + n^2*d - nd) / 2
