Question

1+2+3+4+5+6............+100

Solve by Formula(A.P)

Answer 1

total terms = 100
a₁ = 1
a₁₀₀ = 100

Sn = $\frac{n((first -term)+(last-term))}{2}$

S₁₀₀ =$\frac{100(1+100)}{2}$ = $50*101=5050$

Answer 2

so using formula of AP

n(n+1)/2
so 
100* 101/2
= 50 * 101
= 5050