k+l=m+n;\\ 15+7 \neq 1+20 " alt="b_n=b_1*q^{n-1};\\ b_{15}b_7=b_1*q^{15-1}*b_1*q^{7-1}=b^2_1*q^{14}*q^{6}=b^2_1*q^{14+6}=b^2_1*q^{20};\\ b_1*b_{20}=b_1*b_1*q^{20-1}=b^2 *q^{19};\\ b_{15}*b_7 \neq b_1*b_{20};\\ b_k*b_l=b_m*b_n <=> k+l=m+n;\\ 15+7 \neq 1+20 " align="absmiddle" class="latex-formula">