49²=(50-1)²=50²-2*50*1+1²=2500-100+1=2401
89²=(90-1)²=90²-2*90*1+1²=8100-180+17921
199²=(200-1)²=40000-400+1=39601
38²=(40-2)²=40²-2*40*2+2²=1600-160+4=1444
98²=(100-2)²=100²-2*100*2+2²=10000-400+4=9604
198²=(200-20)²=200²-2*200*2+2²=39204
№251
а. =9а+5в, б. 12х+3у, в. =20а+2b, г. =3а+8b, д. =9х+2у, е. =3а+16х