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Question

Use the product (a + b)(a - b) = a2b2 to evaluate :

(i) 21×19

(ii) 33×27

(iii) 103×97

(iv) 9.8×10.2

(v) 7.7×8.3

(vi) 4.6×5.4


Solution

(i) 21×19=(20+1)(201)=(20)2(1)2=4001=399

(ii) 33×27=(30+3)(303)=(30)2(3)2=9001=891

(iii) 103×97=(100+3)(1003)=(100)2(3)2=100009=9991

(iv) 9.8×10.2=(10.2)(10.2)=(10)2(.2)2=100.04=99.96

(v) 7.7×8.3=(8.3)(8+.3)=(8)2(.3)2=64.09=63.91

(vi) 4.6×5.4=(5.4)(5+.4)=(5)2(.4)2=25.16=24.84


Mathematics
Concise Mathematics
Standard VIII

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