It is difficult to estimate the distribution of XY. Under the condition that X and Y are not independent, P(AB)=P(A)P(B/A) is calculated by conditional probability.
Encyclopedia of senior high school formulas: Encyclopedia of senior high school mathematical formulas: sum of two angles formula SIN (a+b) = SINA COSB+COSABSIN (a-b) = SINA COSB-SINB COS (a-b) = COSA COSB+SINA SINB NBTAN (a+b). =(tanA+tanB)/( 1-tanA tanB)tan(A-B)=(tanA-tanB)/( 1+tanA tanB)ctg(A+B)=(ctgActgB- 1)/(ctg B+ctgA)ctg(A-B)=(ctgActgB+ 1)/ (ctg B-ctgA) angle doubling formula tan2a = 2tana/(1-tan2a) ctg2a = √ ((kloc-0/-COSA)/2) sin (a/2) =-√ (1-COSA)/2. =√(( 1+cosA)/2)=(( 1-cosA)/(( 1+cosA))tan(A/2cos((A-B)/2 cosA+cosB = 2cos((A+B)/2)sin((A-B)/2)tanA+tanB = sin(A+B)/ COSA COSB TANA-TANB = SIN (A-B)/COSA COSB CTGA+CTGBSIN (A+B)/SINA SINB-CTGA+CTGBSIN (A+B)/SINA SINB The sum of the first n items in the partial sequence is 1 +2+3+4+5+6+7+8+9+…+n = n= N2 2+4+6+8+ 10+ 12+ 14+…+(2n)= n(n+ 1) 12+22+32+42+52+62+72+82+…+N2 = n(n+ 1)(2n+ 1)/6 13+23+33