Sum of squares of n numbers is given by the formula $\dfrac{n(n+1)(2n+1)}{6}$
Here n = 20 so,
= $\dfrac{35 × (35 + 1)(2×35 + 1)}{6}$
= $\dfrac{35 × 36 × 71}{6}$
= 210 × 71
= 14910
So the sum of squares of first 35 terms is 14910.