Read the words silently and find one word that is pronounced with the diphthong [ai]. Then write out the words that are pronounced with a short [i] sound and which are pronounced with

13;0;-13
a1=13
d=a2-a1=0-13=-13
S12=(2a1+(n-1)d)/2*n=(2*13+11*(-13))/2*12=(26-143)*6=-117*6=-702

Sn=2a1 + d(n--1)/2   *n 
S{20) = 26+ (--13) *19 /2  * 20
S= 4420//2 => -- 2 210